the senate is the "congress" too. Constitutionally he can give it in written form only. According to the Constitution he can also WAIT since it the wording is "from time to time'.
While we tend to think not it as traditional for the president to give his State of The Union report in the spoken form in the House Of Representatives with a written form given in advance it's not always been done this way and doesn't have to be.
Oh, and I've not found anything that says the president's report is "congressionally mandated" to be given on the 29th. What I did find however is although an invitation was issued, neither the Senate nor the House has passed the necessary resolution yet. Perhaps this is what the writer is thinking of but there's no '"mandate" yet.
WaPo has a very informative article on the history of the State of the Union. If needed I suggest using 'private browsing' to view the rest of it but below are what I consider the major points pertaining to this... 
https://www.washingtonpost.com/politics/congress/q-and-a-does-trump-have-to-give-state-of-union-speech-no/2019/01/16/c7fafabe-19b6-11e9-b8e6-567190c2fd08_story.html?noredirect=on&utm_term=.f5b278100a02
Does Trump have to give a State of the Union speech? No.
WASHINGTON — The U.S. Constitution mandates in Article II, Section 3 that presidents “shall from time to time give to the Congress information of the state of the union, and recommend to their consideration such measures as he shall judge necessary and expedient.”
But the Constitution offers no specifics on how that should be done. Indeed, the modern State of the Union address — the pageantry, the televised address and the agenda-setting message — is a far more recent phenomenon. And the practice of delivering an in-person address before a joint session of Congress has not always been the norm.
Here’s a look at the history of the State of the Union address as the White House weighs its response to House Speaker Nancy Pelosi’s suggestion that President Donald Trump delay this year’s address or give it in writing because of the partial government shutdown:
-snip-
Q: Does it have to be a speech?
A: No. For Thomas Jefferson’s first address on Dec. 8, 1801, he sent written copies to both houses of Congress to be read by each chamber’s clerks. Jefferson wanted to simplify what he believed was an aristocratic imitation of the British monarch’s speech from the throne, which he thought ill-suited for a republic.
The practice of sending written copies to Congress continued for more than a century, when Woodrow Wilson resumed the tradition of delivering the annual message in person on April 8, 1913. He’s also credited with transforming the speech from a report on executive branch activity into a blueprint for the president’s legislative agenda for the year.
-snip-
Q: Has the speech ever been postponed?
A: Yes, there have been several instances — though all appear to have been initiated by the White House, historians say.
Ronald Reagan’s 1986 address, for instance, was postponed after the Challenger space shuttle exploded in flight on Jan. 28 of that year. And in 1944, Franklin D. Roosevelt was scheduled to deliver his address on Jan. 11 but instead transmitted a written message because of his poor health.
___
Q: What about during a shutdown?
A: The State of the Union address has yet to coincide with a full or partial government shutdown since the beginning of the modern budget process in the late 1970s.
The closest the country has come? Trump’s 2018 State of the Union, which was delivered on Jan. 30 — the week after a two-day shutdown that ended Jan. 22. In 1996, Bill Clinton delivered his State of the Union several weeks after the end of a 21-day shutdown that had previously been the country’s longest, offering to “all of you in this chamber: Never, ever shut the federal government down again.”
-snip-
Q: Can’t Trump just show up?
A: Nope.
Anytime a president comes to speak, it must be at the request of Congress, said Donald Ritchie, the former historian of the Senate. “It’s a very inflexible arrangement,” he said.
A resolution agreed to by both chambers specifies a date and time for a joint session of the House and the Senate “for receiving such communication as the president of the United States shall be pleased to make to them.” [*There hasn't been one passed yet.]
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Q: Is there a State of the Union speech every year?
A: No. Recent presidents — Reagan in 1981, George H.W. Bush in 1989, Clinton in 1993, George W. Bush in 2001, Barack Obama in 2009 and Trump in 2017 — have chosen not to deliver official State of the Union addresses during their first years in office. Those speeches would have come soon after their inaugural addresses. However, many, including Trump, have delivered major speeches in front of Congress that have had the feel of the State of the Union without the title.
“It’s up to the president,” said Ritchie, “whether or not he wants to come at all.”
-snip-
Q: Could Trump choose an alternative?
A: There’s no reason why not. Trump could deliver a speech from the Oval Office or organize an alternative event on Jan. 29 — the date Pelosi had originally invited Trump to deliver his address before a joint session of Congress. The White House also could choose to delay the speech, as Pelosi has suggested. It was unclear how the White House planned to respond.
But Ritchie said it would be to the president’s advantage to find a way to deliver the address.
“You don’t want to waste a dramatic moment,” he said.
...And goodness knows 'Don the Con' Trump loves a "dramatic moment".
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