I see value in being uncompromising (look at Richard Stallman in Free Software). I also see value in giving a little in the right areas for a net gain.
I don't think McBride thinks that this is the ultimatum given at the moment (100% vs 50% of trans people die), that would explain why she is willing to compromise.
If she did see it that way, she would probably do the same as you.
Her wanting to compromise at this moment does not mean she would compromise in the worst of moments.
One could make similar statements (and some do), about voting for the democrats over a better farther-left party. The defenders of voting Democrat would likely tell you the same thing: "Its our only chance at winning."
Are you voting for genocide if you compromise and vote for the Democrats? Or are you merely making the best of a broken system?
You decide.