As I understand the post, its idea is that a rationalist should never "start with a bottom line and then fill out the arguments".

I disagree. The idea, rather, is that your beliefs are as good as the algorithm that fills out the bottom line. Doesn't mean you shouldn't start by filling out the bottom line; just that you shouldn't do it by thinking of what feels good or what will win you an argument or by any other algorithm only weakly correlated with truth.

Also, note that if what you write above the bottom line can change the bottom line, that's part of the algorithm too. So, actually, I do agree that a rationalist should not write the bottom line, look for a chain of reasoning that supports it, and refuse to change the bottom line if the reasoning doesn't.