You can read jaytea's post
to understand why using noop with filter is a problem, and it's not about /!noop at all
$noop() doesn't exist, so if you have the identifier warning on, mirc will report an error, this is the same for any non existing alias, he could have used "thidoesnotexist", when you don't have that warning on, any non existing identifier returns $null.
!noop prevents mIRC from displaying the * No such identifier message (is it like ~ or a bug?)
/!command only search for the builtin command and $~ident only call the
built in identifier, so as a side effect, it won't produce the error if you have the warning on.
Jaytea's post also describe the fastest way to use filter to get $filtered filled, it's to use "~~" as an alias.