It’s less a vote for Trump and more a lack of a vote for the party that can realistically defeat him. I wish 3rd parties were a viable federal election option. But they aren’t yet to a point where they can realistically win in the US.

So the sentiment that not voting, or voting for a guaranteed loss is a vote for the worse of the two parties with a chance to win isn’t entirely without merit, it’s just poorly phrased. It means that if you fail to vote for the person that can win over the worse candidate, you have given the worse candidate a better opportunity to win.