This indeed looks like a FUD statement, implying that they can have an infinite amount of potential vulnerabilities. Realistically though, writing parsers that do not yield control of your whole device is not that complex. The people exploiting iOS zero days can certainly do it.
You're not wrong at all, but if they're shipping these garbage ancient versions of ffmpeg, there are likely oodles of other bugs lurking around. And, if Cellebrite acts like most other companies who've had their awful security exposed, they will fix only this bug and leave everything else.
the signal are capable for finding more exploit with more time. important piece is that exists now a reasonable doubt on data from the celebrite, so it are not so good for evedince.