What is Containers/com.apple.mail suppose to look like?
The short answer is yes, I believe your ~/Library/Containers/com.apple.mail is what it is supposed to be.
Just like you, I do have ~/Library/Containers/com.apple.mail which Finder presents as one of 6 subfolders called Mail.  Mine has permissions like yours and same content including all the symlinks.
So don't worry, everything is (most likely) just as it is supposed to be.
The more worrying answer is that Apple Support staff mostly do not have the faintest idea about the changes in Big Sur.  I have come across other reports where Apple Support seems to be reading from their Catalina support notes.