The optimal thing for them to do would be to build the framework and ship partially corrupted JPEGs that don't actually do anything nasty to Cellebrite. Cellebrite can verify that the machinery is there (not a totally idle threat) but no one can prove that Signal has actually done anything illegal. Cellebrite then wastes a bunch of times gathering and analyzing the files without actually learning anything from it. They also get more incentive to start finding and fixing their software's vulnerabilities, which throws off work schedules.
And Signal can develop a few land mines to deploy at any time, and just... hold on to them for a rainy day.