They are the same. Foundry was founded by a GW guy, and many of the old citadel historical ranges were reused.
Bryan Ansell. Citadel guy, IICR, before they were part of GW. Even after the split to Foundry, I think they still shared sculptors.
I don't know about now, but I remember buying Foundry figures that still said "Citadel" or "Games Workshop" on the bottom years ago.
A couple duplicate ranges that I remember off hand are that Citadel Brets became Foundry Medievals, Barons Wars, etc., Norse became Vikings, and Nippon became Samurai.