> the existing check in `do_help` didn't catch this case because https://docs.python.org/3/library/sys.html#sys.flags only reflects command line flags, rather than whether or not it was actually optimised.

Precisely. Your check is much more appropriate for what the actual issue 
is going to be, though keeping the existing check there means we can 
provide a more specific error message when we *know* that the user 
deliberately chose that behaviour.