I always liked the term "search engine client" better (vs 'metasearch engine'). In essence it is a product that can connect to different search indexes.

An "email client" does exactly the same thing, connects to different email servers and we do not call it "metaemail".

edit: just realized that with the current hype around metaverse, 'metasearch' will probably be more appropriate for something searching the metaverse in the future.