Republicans joking "Don't try this at home" are being complete jerks and endangering people.
That phrase is used when an expert demonstrates something successfully in a carefully controlled (non-home) environment. It's telling people not to do something that experts can do. That's not what injecting disinfectant is.
Experts can't inject disinfectant into people and get good results. "Don't do this at home" doesn't apply here. It's Republican defenders of Trump trying to twist Trump's idiotic statement into a sneaky "half-joke." But that half-joke gives a dangerous message. "Don't do this at home" implies experts like doctors might be able to do it. And some people probably will do things at home that they are lead to believe experts can do.
The right thing to say to the American People about injecting disinfectant is, of course, "DON'T DO IT!" Not stupid, sneak, semi-jokes about not doing it at home. It's not funny, Republican Trump supporters! You're not funny! You're jerks.