I'm being facetious mocking the notion that by not voting for Biden one is voting for Trump. I understand the intricacies and failures of the US electoral system, and I understand that in a two-party race, by not voting for the one person, you're making it harder for them to win. However the mistake in the logic is pretending as if this is some extravagant circumstance.
It is correct that by not voting for Biden he will get less votes, and thus have a harder time winning. The same goes for Trump, by not voting for him he gets less votes and will thus have a harder time winning. The same goes for a third candidate. That's the way elections work: You need votes to win.
If Biden wants to win, he should do something to get votes.

In other words: It is correct that by not voting for A, B will be more likely to win, since A will have less votes. A should probably do something to get more votes then.

Edit: Thank you for your well-thought out input and answer to my question however. I do very much appreciate your sincere response <3