He’s not refusing to acknowledge spoilers exist, he’s saying that the whole premise is that they are voters who would be voting for the larger party if their policy positions were adopted or engaged with by the party they’re “spoiling”, and yet when the e.g. Democratic Party just refuses to engage with any policy changes and thusly doesn’t gain those voters who were available to them, they turn around and blame the voters, when it was literally a choice they made to decline engaging with their positions. He is pointing out that it is the party choosing to stick with their corporate-backed positions over gaining voters (i.e. over winning).

They either don’t understand, or actively refuse to engage with, coalition-building.