The argument is quite simple while it carries an assumption.
If you have 3 options, and depending on how you want to frame it, one is outlandish or the other 2 are simplify more similar. You have following issue.
In this example A and B are similar and C is the outlandish one.
Let’s say: A has 15 votes, B has 3 and C has 17 votes.
Then C wins while it would be reasonable to assume that B voters would have chosen A over C, as A is more similar to B than C. So now the A and B voters get together and talk about the situation. A voters argue that A had historically far better results than B and B voters should have expected A to get more votes than B, and as B voters prefer A over C, B voters risk that C wins as A is missing the votes from B voters. So while not voting for C, B voters voted in a way that is unlikely to result in B winning, while hurting A winning chances as they didn’t vote for A, which results in C requiring less votes to win and could help C in winning
So in other words, if not C, is a shared interest of A and B, voting B is expected to reduce the amount of required votes for C.
If C needs 18 votes and a “not C” voter votes B, A cann’t reach 18 anymore, ofc B can reach 18 but historically B never got close, so effectively C requires 1 less additional vote to win, just like when someone would have voted c.