There are two components though. One is the ability to choose the party, the other is the pressure on a party to adjust so that it appeals to enough voters to elect it. If a party doesn't need to adjust to get some votes, as in proportional representation, then we (potentially) end up with coalitions that live or die based on narrow issues, which is what the original post I replied to alluded to. On the other hand, if you have two parties that fight over the votes, you come closer to having parties that optimize for broad appeal.

There are lots of problems with democracy (just fewer than the other potential systems as has famously been said). Imo a two party first past the post system can actually help regularize the will of the people by forcing parties to align with actually electable platforms and not dig in on single issues. Lots more to say about that, I just want to counter the usual rhetoric about how proportional representation or similar systems are somehow automatically better

This isn't quite right. U.S politics is much less party-centric than it is in many other countries. Basically, though you've got a vast majority of candidates for X or Y office who are either democrats or republicans, their platform tendencies can be quite diverse, even though they often go against the grain of their own party's nominal positions. It's somewhat subtle, but this detail of U.S party politics makes for a strong multi-party diversity of candidates even though there are formally only two parties that mean anything for votes.

This is how you can have, for example, a surprisingly moderate republican senator like Susan Collins, sharing the same party with someone like Trump. And then also in the same party, a Rand Paul type.