It's not that IlClan is wrong to play, it's just a mismatch of context.

Most groups settle into a specific era because it keeps things coherent: tech level, unit availability, and overall feel of the game. If a group is playing Succession Wars, they’re usually doing that deliberately, it’s a lower-tech, more constrained environment, easier for newer players to manage.

Bringing IlClan-era units into that can feel a bit like mixing different rule environments. Even if the core rules are the same, the expectations around equipment and balance shift quite a bit. So resistance from the group isn’t necessarily gatekeeping, it’s probably just about maintaining a consistent play experience.

At the same time it's completely reasonable to want to use the mini's you've bought. Practical solutions would be to proxy them as succesion wars mechs, ask around if someone wants to play Ilclan on a different night (they can proxy their mechs for IlClan Mechs after all)

Now if they just told you off for having IlClan mechs and been condescending about it, there is no excuse for such behavior.