I see a few potential ways someone might disagree:

• Any kind of political revolutionary who doesn’t consider reformism a viable strategy and doesn’t believe their goals are possible to achieve through voting. e.g. even if every US politician were to be replaced with Mamdanis overnight, the US would never progress beyond “capitalism with New Deal-style concessions” without tearing down and rebuilding the entire state from the ground up. See also: how aggressively the political machinery across most of the west mobilizes to discredit and/or oust any prospective politicians with remotely socialist or anarchist views which could threaten capitalists’ position.
• More relevant to the original meme, trans people (or perhaps other minority groups) who have been thrown under the bus in the past by politicians who claimed to support them, only to pivot after being elected, as a number of Democrats have done recently.
• Another train of reasoning I’ve seen: trans people who fear the potential consequences of electing a Democrat with a neutral or anti-trans position. Paraphrasing an explanation I’ve seen, the premise is that while a Republican president (or whatever other position) will almost certainly enact immediate negative changes for trans people, there is still the possibility that a pro-trans Democrat could be elected in the future. However, if an apathetic or actively hostile Democrat were to be elected (e.g. Newsom), it would set trans people back for decades, as the Democrats (always looking for ways to pivot right to appease some hypothetical median voter) would see this and conclude that trans rights are an effective compromise in exchange for winning votes. They would therefore continue to push candidates who act actively against the interests of trans people, making forward progress for trans rights an impossibility for many election cycles.

I should note that I’m neither an American nor would I consider myself a subscriber of any particular political ideology beyond an assortment of vaguely leftist beliefs. This is mainly just me paraphrasing some sentiments I’ve seen/heard which would probably disagree with the comment in question.