Currently MediaWIki PHPcs sniff FunctionCommentSniff only validates @return annotations of methods with a body.
It does not complain about annotations like @return boolean or @return integer that should be written @return bool and @return int.
The relevant line in FunctionCommentSniff is: https://github.com/wikimedia/mediawiki-tools-codesniffer/blob/5f813f727ee47d5c90981b44e82f0c5a0263e933/MediaWiki/Sniffs/Commenting/FunctionCommentSniff.php#L198