fish, the friendly interactive shell, is a commandline shell intended to be interactive and user-friendly.
fish is intentionally not fully POSIX compliant, it aims at addressing POSIX inconsistencies (as perceived by the creators) with a simplified or a different syntax. This means that even simple POSIX compliant scripts may require some significant adaptation or even full rewriting to run with fish.
I think that’s the point of the comment
Exactly, use the shell you like (nushell in my case), write POSIX scripts for maintainability, and use shebangs so you don’t have to think about it.
If you like fish but don’t use it as your login shell because it’s not POSIX you’re missing the point of the shebang