A contributor's license agreement would overrule the terms of the employment contract, assuming they set one up properly.

Why would that be an issue?

Because it would mean any contribution by AnkiDroid's owner to AnkiDroid would be considered property of his new employer, AnkiHub.

If it's still open source it doesn't matter who owns the copyright.

Not strictly true afaik? If you own the copyright to the entire codebase you can relicense at will to a different license. (that's what CLAs enable among other things)

Not sure whether you'd still be entitled to the source code under the previous license then.. can a copyright owner revoke a previously issued license to the code? Haven't heard of it, but wouldn't surprise me if it's legal.

I am aware, but I don't think that's what OP was talking about.

You can't revoke previously issued licenses (unless the license allows it, which it doesn't in this case).

Sure, you can change the license, but the old license still applies to the code as it was before you changed it. Assuming you're using a legit open source license the first time around, nothing changes regarding how you can make use of the old code; all they can do is make it harder to find (close the repo) or harder to make use of (squashing/flattening the commits to make it impossible to get the correct historical version), both of which are trivially bypassed by using a third party fork or source release.