No, it doesn't, the '!' command prefix is only about processing the built-in command rather than your alias:
alias filter echo -a you're a filter!
If you have this, you can do /!filter to process the built-in command but /filter will echos "you're a filter!" to the active window, there is nothing about recursion here.
Recursion occurs only when you are calling the same alias from itself:
alias filter echo -a you're a filter! | filter
As you may know, since mIRC doesn't support direct recursion, rather than throwing an error when trying to execute /filter from the filter alias, it executes it as the built-in command, which here, will report "/filter: invalid parameter", but if you were not using an alias's name which is a built-in command, it would have sent the command to the server.
Allowing direct recursion means calling /filter here would keep printing "you're a filter" to the active window" because the /filter call at the end of that filter alias would just call itself, it wouldn't call the built-in /filter command and, since that behavior would be different from the current behavior, you would be breaking compatibilities.
