It also uses the name “Portal” in a way that feels official, too. If it said “Revolution: A Portal mod” it’d be fine.
How does it “feel official”? It’s just what a lot of mods do:
- Portal: Prelude
- Portal: The Flash Version (though this one is a standalone game)
- Half-Life: Echoes
- Half-Life: Caged
These are just a few I can think of, you can find more on Steam and ModDB.
Without any prior knowledge if a trailer for Portal: Prelude were released today it would be equally as confusing. The flash version wouldn’t be since it would be strange for Valve to make a flash version of anything officially, so that’d set off flags, the Half-Life ones likely would be confusing as well, purely from a “released a trailer with a very clean logo and the word official in the title” standpoint, not really a general product naming standpoint, but that series is more dormant, so maybe it’s less likely to be confusing.