Nobody's missing the "important point" because everybody who's been on Hacker News for five minutes has heard Paul Graham make that point. Indeed, people have made jokes about startup business plans for decades. Literally everyone knows that those things don't matter for super early stage startups.
The tweet, however, and very much like many of Paul Graham's latest essays and tweets, seems to purposely elude any context, make a very generic a debatable statement, and assert it with complete aplomb. For someone who just wrote an essay about putting ideas into words, if he can't see that this tweet will be interpreted as "balance sheets are useless", he needs to put in a bit more work.
That's what people react to, because the only conclusion is that since he doesn't want to tweet bromides, he must be trying to say something "controversial."
It's really bordering on trolling, which for a man with his age and reputation is kind of cringy, embarrassing, and frankly disappointing to people who enjoyed his past stuff.