No, collecting UDIDs and using (multiple) standard developer accounts.
With TestFlight you don't even need to collect UDIDs or even email accounts anymore. And that's 10,000 users per app.
TestFlight requires going through a partial app review processes.
True. I'm surprised the enterprise app doesn't involve the same review.
It shouldn't involve the same review. Apps internal to a company might break some rules set by Apple, but are agreed to by the employee in question via their contract or consent. The whole point of the enterprise distribution system is to easily distribute apps inside your organization.
Facebook clearly abused the distribution system here.