That's not true at all. Most vendors, if you make them specify it in a contract, will provide you with a way to export the hashes and some vendors support importing users with existing hashes. Okta in particular will import existing hashes.

My company moved to Okta from a home grown solution in 6 months and I suspect after the work we put in place to facilitate that would allow us to move somewhere else in even less time as long as they supported importing hashes.

There's a big gap between "never build yourself" and "letting other people handle it". Yes, you should have full control over your auth, run it yourself and keep the data - but you still should not build it yourself, you should use well-established solutions/libraries built by others instead of trying to figure out e.g. what's the proper way to salt passwords.

Exactly, how are all the Auth0 customers feeling about that acquisition?

Why can't you switch vendors?