RiotX? I don't remember that. There was a TelegramX, maybe this is what you mean? However there was a couple years that I didn't use it much so perhaps I missed it.

But it did go from Vector -> Riot -> Element and various UI and branding changes along the way yes :) Part of this is also that the app was not really "finished" back in the Vector days. It only really came into its own during the Riot age. For example E2E encryption didn't yet exist at the start, and the underlying support in Matrix was missing as well. It's still a protocol and toolset that's evolving rapidly.

However the benefit of Matrix is that you can choose any client you want, there are several for mobiles.

The app that is now the Element Android app co-existed with the old Riot app for over a year as "RiotX". The rebrand and app changeover were timed together, so they renamed the RiotX app to the Element app and replaced the old Riot app with it in one go.

RiotX existed as an experimental client alongside the original Riot, the intention was always to replace Riot with RiotX and this was made very clear (The original java implementation sucked, although what has become Element still has its many many problems)