If you are talking about the digital file itself, the short answer is: NO, because a map file is not a tangible thing like ie. a book that you can buy in a store. A map file is just a block of information that can subjected to a license contract (ie. i disclose this much information and you do X, Y, Z) giving birth to contractual rights (ie. the Blizzard's EULA says you can't use cheats while playing the game). As any block of information, a map file can give, sometimes and under some circumstances, and to some subjects a handful of other rights (ie. Copyright Law says that only the author can publicly display a protected work such as the object code of the WC3 Frozen Throne Game). This means that, technically speaking, any modder can subject to a license their respective map, because the "owner" of a digital file is the actual de facto owner of it. If a person different from the de facto owner of a digital file (ie. a program's executable, a digital photo, a digital asset) is interested in acquiring such block of information (ie. potential players of a the game/mod) then they must comply with X,Y,Z. Such is a license contract.
If you are talking about IP rights, fundamentally Copyright rights then, the short answer is also and most likely: NO. Atleast not "now". Most copyright systems are based on this general rule: the author of a protected work is the owner of such rights at the moment of creation. Note that i'm talking about "owner of rights" not "owner of the map" (see above).
Exceptions to this rule are prescribed by copyright law itself and often extend to work for hire cases. Let's say i have a game company called Buzzard and this company contracted a handful of programmers (work contract) to code for a game called War-Crack III: Recycled. For these cases, and most often that not, the law says that any copyright rights are born (spawn if you will) directly in favour of Buzzard unless the contract implicated (in this case the work contract) says otherwise. On most systems these exceptions are redactated in such a way that they CLEARLY attack the "ownership" concept itself (which is wise), so in our example the original owner of the protectable work is Buzzard and not the programmers (ie. the work contract wasn't an assignment -transfer- of the rights of the programmers in favour of Buzzard).
Now ask yourself: is this EULA or the Custom Game Acceptable Use Policy a work for hire hypothesis? If not, then what rule we apply? Yeah the general rule.
So what is the actual meaning of the "Blizzard owns your IP clause". Enforceable at all? Do modders have actual interest or copyrights in the first place? What are the odds? Since i posted my opinion so many times i will, from now on, only answer such question privatedly and charging some money. Sorry.
Finally, you must note that Copyright doesn't apply to ideas, game mechanics, code algorithms, plain facts and such, but rather the creative use, if any, the author does to those respective ideas, concepts, facts etc. Every journalist, blogger, reporter, can speak about coronavirus but if you want to display their respective column on your own blog, assuming they didn't mechanically presented the topic (read: they were creative thus "spawning" a handful of rights) you will need a license because such "column" is protectable by copyright. Because of this, copyright will hardly ever apply to plain names, denominations and such. So in the end: Blizzard doesn't have any legal interest on game mechanics, concepts and such but so do modders and game developers in general.
The only thing that can come close are patent rights, trade secrets and unfair competition cases. For denominations and names a trademark could apply (observing the recquisites of course). Also, most patent systems preclude "games" (scholars speak about these "not being subject matter" of a patent) from being patentable in the first place. Still, be wary that ie. the map triggers or a program code is not technically the game itself or its mechanics, so you could say that a game like Warcraft III or any digital game could have some patented algorithm or method inside its code, but the odds are slim.
EDIT: put some examples.
