Not quite.

They behave like IO::File objects (since 5.12).

$ perl -e'STDOUT->foo'
Can't locate object method "foo" via package "IO::File" at -e line 1.
[download]

They do obey inheritance.

$ perl -e'use IO::File; IO::File::say( *STDOUT, "ok" )'
Undefined subroutine &IO::File::say called at -e line 1.

$ perl -e'use IO::File; IO::Handle::say( *STDOUT, "ok" )'
ok

$ perl -e'STDOUT->say( "ok" )'
ok
[download]

(IO::Handle's docs is merely saying IO::File isn't a derivable class since My::IO::File->new will create an IO::File "object" instead of a My::IO::File object.)

And the issue raised wasn't why it works; it was about where it's documented. I could not find it either.