Answer
'Base method should be overridden and not overtaken' is a P2 Static Code Analysis (SCA) error, and it should be treated as an error, not a warning — the colleague seeing it as only a warning almost certainly has SCA checking disabled via the 'NoErrorOnStaticCodeViolation' feature toggle under Project Properties > General > Feature Toggles in their Developer Studio instance. That toggle should be left unchecked so SCA runs and enforces this as a real error; disabling it just suppresses a check that's there specifically to catch this kind of unsupported overtake before it causes runtime/deployment problems.