https://whosthat.osmz.ru/?id=18876044 returns user properly
Searching 18876044 directly of course returns 'no users found' because it is only username search.
It is understandable it is less common to have uid of user and the user can already enter the uid in the url.
However, it would allow the user to easily specify they are searching uids, ie. if they have a list of them / earlier search results (assuming it is not something they will bookmark) on hand. (easier than searching many character long name)

MapComplete allows interchangibly searching for wikidata qid and name of species (as it is an editor which might have existing values it should maybe not override and propably for ease of implementation it uses a separate search input to do so)