"Who's to say that speaking English with an American accent is the correct way?"

If you're trying to found a company and get venture funding from Americans, you need to be able to communicate well with Americans. No one is saying there's a correct way to speak anything, but it's obvious Paul Graham is talking about founders going for an American market.

"Also remember communication is 2 way. If you can't understand someone due to their accent most likely they cannot understand you due to your accent."

That's just straight out wrong, sorry to say. There are plenty of cases of one way unintelligibility in ALL parts of the world, in many languages. English is no exception, just consider the stereotype of American tourists being confounded by Australian or England-English.