Because it's a non-partisan point. Pretty much in every state the party in power gerrymandered--and the reason you gerrymander is precisely the rationale given in your excerpt.
We're just not used to seeing it stated so bluntly. Or, put another way, honestly.
Only recently have Democrats in some states pushed for a non-partisan (which often means not a priori partisan) redistricting process. That doesn't dispose of 150 years of gerrymandering, however. Some people like to act like the past is always fully present. In this case, the past isn't fully past--in some states (D) still gerrymander, and on DU you hear frequent calls for gerrymandering to exclude a party from office. Ahem.
Look up the 2002 28th New York Congressional District. It goes from SE of Rochester, NY, includes the city of Rochester (D), a bunch of (R) thinly populated area along the Great Lakes, circles south through Niagara Falls (D), Tonawanda (?), and gets part of Buffalo (D). That monstrosity's been struck.
The court threw out the TX redistricting map, though, because (D) and (R) are correlated with race, so by helping (R) they hurt many non-whites and "diluted" their vote. The correlation isn't exact, so the revised redistricting may have hurt (R) but a couple of those (R) politicians placed at risk (placed in (D) districts?) were Latino. They didn't find that the redistricting was racist; they said it bore the "mark of racism," a slightly different kind of thing.
Disparate impact is often deemed to be proof of racism; no need to show intent or motive. Even if you can prove you're innocent of actual intent, if the evidence can be interpreted as the result of racism some laws require that it be assumed the evidence demonstrates racism. Just sometimes. It makes for a nasty asymmetry in the law.
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