From what I’ve seen, it’s not that they’re “young” galaxies, but that they shouldn’t have had enough time to develop if the universe were truly so crazily homogenous from the big bang. It doesn’t necessarily disprove the big bang, just means the universe might not be as “smooth” as previous assumptions.

Any scientist worth their salt should be readily able to admit it was always an assumption, just one that proved congruent with observations until now.