Architects are primarily judged on the aesthetics of a building, as well as the functionality, and there may have been a difference of opinion and/or vision on how the stadium was to be built. Perhaps the coronavirus has forced the club to cut costs somewhere. In any case it's quite frequent for the architect to fall out with the client/and or main contractor.
The architect still has an important role to play in the project until completion. A large part of the design work will have been done already but it's very common for projects to commence before the full design has been completed. The architects will have a team of people looking at the technicalities and coordinating designs with the main contractor and subcontractors later down the line.
It could be that Meis were only down to do the concept design and another architect has been appointed for the actual build.
Personally I think he should be keeping his mouth shut out of respect to the client and the project. I think he's shown a lack of professionalism by putting this personal thoughts on the project on social media. My company (a developer) would be fuming with the architect if they did similar, and rightly so.
