Fighters are judged only by what they achieve in their time. Yes, a fighter who struggled with Braddock and Galento would struggle with Joshua, who would beat them both on one night. It doesn't make Louis less great, though.
It's more likely that Louis beats both in one night than him losing to either. They're Louis' bread and butter, especially Zhang.
Both are tailor-made for Louis. Zhang despite his power and size is slow as molasses and has no stamina. After a couple of rounds Louis would have him on his back. As for Joshua the hand speed, combinations, and counters that Louis brings to the table are all wrong for Joshua.