Impossible, yet accomplished by thousands and thousands of small business owners every month.
Similarly, a small business owner might call themself a CEO.
Neither is wrong per-se, but when we're talking about CEOs who have hired a team of executives to run their company, it's most reasonable to assume that GP was not talking about such a small business.
This also doesn't quite acknowledge that even if you can do all of those functions [in a small business or one-person company] it does not mean you're actually good at those jobs.
It's an appealing thought, but really they are doing a different job. Vast majority of those thousands of small business owners couldn't effectively step into one of those roles at even a mid-sized corp, let alone all of them.