`dom.workers.serialized-sab-access` is the flag they've put in so that way they can disable concurrent execution of JS threads that share memory in case a novel cross-process attack shows up. Spectre is purely an in-process attack and the whole article that comment is attached to is about the work they did to enable shared memory while defending against Spectre. `dom.workers.serialized-sab-access` does not affect Spectre. It appears to be intended for preventing a novel cross-process attack from leveraging shared memory in other processes into becoming a high-resolution timer.
Also note that threads that have access to shared memory in Firefox 79 also have access to the full high-resolution performance.now(), and flipping dom.workers.serialized-sab-access doesn't affect that.
