I disagree about whether this should be fixed or not.
It's definitely a bug. If you ask whether a bind mount destination is a mount, it should return true.

I wrote a logic that does bind mounting. The logic cannot use ismount() because it does not work for me, it would keep remounting the same bind mount.

Not fixing this just causes people to write even less portable hackery on their own. I have to parse /proc/mounts or something manually, which obviously should be the job of ismount().
Python has a lot of platform-specific underlying implementations anyway.