I suspect that it's the "just", for reasons that sibling commenters have discussed.

But the original comment might also wrongly come from the perspective that a given developer ought to tie themselves to a particular project by creating a business around it. Many projects are created and/or maintained by developers working on a broad number of projects, so total productivity might decrease if developers were settle down with only one or two projects which they're able to build a business around.

Yes, I think so. 'just start a company and make money' is exceptionally non-trivial. 'just' is usually used to infer the subject is easy. "It's not hard; just do it!"

That type of phrasing can also be used sarcastically or meanly, so people may have interpreted it that way. "Why can't you do this? Just make it work!"

Because it's free as in freedom, not free as in price. Most popular free software is written by employed developers working for companies.