I might be incorrect, but I think they are trying to suggest that no election is a standalone choice where the only downstream effect is that one candidate wins over the other. Each election outcome effects the elections that come after it. If a moderate republican won last time they might try to run a more conservative one next time to test the waters. If a progressive dem lost the dems are likely to use that as a reason to not run a progressive next time.
They may be suggesting that voting for an anti trans dem now opens the door for more anti trans or anti-XYZ dems in the future. Do we want to get to a place where the Democratic Party is comfortable running anti choice candidates or anti public school candidates? Basically the idea is to signal to the party that they will not ever win with this message, even against fascists. Along the same line of thinking, but as a rebuttal to what I'm assuming is their conclusion, an R in office might change policies or do harm that a dem may not be able to reverse, regardless of if they win next or not.
I'm not suggesting people not vote for him, and I'm not suggesting vote blue no matter who, I'm just adding some context for how I interpreted the comment. I personally am a vote blue no matter who person so far. Maybe it makes me a coward, maybe it makes me pragmatic, maybe both. I do think I understand trans people withholding their vote if this is their candidate though. I won't blame them for standing up for themselves, since the rest of us seem willing to throw them under the bus.