That is not true. Voting for someone other than Biden or Trump tells the major parties the one thing that matters: "If your candidate sucks, we won't vote for them." That is the only way you can show them at the ballot box that things can't continue as they have been. They have to lose and continue to lose until they change or cease to exist. Neither major party is doing much of anything for me, so I don't care if they both go the way of the Whigs.
By "sucking it up" and "voting for the lesser of two evils" you only assure that we'll get more evil in both major parties in a race to the bottom that we've been on (IMO) since the 1980s.
A vote for Jill Stein is NOT a vote for Trump. That is physically impossible.
I will not vote for a candidate that is openly supporting genocide and is demented (Biden) or who will probably openly support the same genocide and is a moron with the attention span of a gnat (Trump). Even RFK Jr. who I like on several issues is a no-vote because he STRONGLY supports the genocide in Gaza.
Every candidate starts every election with 0 votes. No votes are "owned" by a candidate. Therefore I can support Stein and it doesn't "add a vote" to Trump.