It's not just the backtracking the parallel runway that causes issues here.
If the problem aircraft was surrounded by airport fire vehicles as indicated
above, then the airport's fire cover may be degraded to an extent that other
operations cannot continue until they are available again. It doesn't matter how many runways you have - if all the fire services are busy, there will be no take-offs or landings
What you say is true, but the issue today for why the 4 aircraft diverted was purely down to landing delays due to backtracking. Therefore there must have been adequate fire cover to continue 'normal' operations (normal in the sense than no aircraft were refused due to no fire cover)