If nobody trusts either party to keep up their end of the bargain, why not solve it with licensing?

Isn't this the exact point of copyleft licenses?

Relicense PebbleOS as GPL, relicense Rebble as AGPL.

Problem is then solved, no?

Almost. The excessive CLA had to go too (or be replaced by a DCO).

CLAs may have a place, but as long as they hadn't planned on a bait and switch all along they wouldn't need a CLA literally copied from Oracle's playbook.