Concealed toilet cisterns attached by screws to dry wall. Are they secure?

I've noted several post-2000 build apartments that have concealed plastic toilet cisterns that are secured at the rear to dry wall by 4 screws. They are lever-type cisterns, so there is also a toilet lever tube (either metal or rigid plastic) connecting the front-right edge of the cistern to the exterior tiled wall. One side has an inlet for a flexi hose, the other is connected to the overflow.

The screws are arranged horizontally along the upper rear surface of the cistern.

I was reassured by an experienced plumber that these drywall-secured screws don't move and even one screw is enough to hold the cistern in place. I don't have any reason to doubt him given that not one of these cisterns have become loose over 20 years. However, should it come to replacing these cisterns, should we try to secure these cisterns to something other than drywall?

I was wondering how secure such cisterns are, how much weight they can hold and if it's normal practice to secure cisterns this way. Should a timber framework be fitted for replacement cisterns?