i definitely get i could do it with that enumeration method, i just me logically if there is a file called test.txt then $session.FileExists(./test*) should be true, because that file "does" exist.

The FileExists method was not intended to support wildcards.

You can easily implement wildcard testing using Session.EnumerateRemoteFiles:
https://winscp.net/eng/docs/library_session_enumerateremotefiles

Shouldn't the .FileExists() method on the Session object support wildcards?

Similar to in powershell to like

Its difficult to test if a wildcard is used, you'd have to test if a wildcard is in the string, then parse the directory from the path, the enumerate, then see if its greater then 1, i guess.