The reason I don’t agree with this - and to be clear, I’ve never reached Gladiator so this is my perspective and I admit I could be wrong - is that in each season it may be easier or harder to hit gladiator.
You may see someone with a Season 1 BFA Gladiator mount on a monk and say “oh wow, it was really hard to hit Glad on Monk that season, good for them” In a similar way that you would look at AOTC on Kil’Jaeden as more impressive than Sarkareth.
EDIT: I forgot about the change that made all Glad mounts account wide. This argument could still be true if one season were particularly more challenging than another.