The
Ultraman Wiki has a Multiverse Map (2017), probably from a Japanese fan magazine. It does show Cosmos as its own seperate universe:
Okay, thanks. I wish I'd done better in Japanese class back in college so I could read the text.
I have only seen the three movies and the first episode of Cosmos so far, so I can't say whether this is adressed in the show itself, but other installments of the Ultra Series have had references to legendary Ultras in the ancient past, most notably Ultraman Tiga. So, it might be a case where in this particular universe, the Ultraman of legend is still a fairly known figure in the present society. On the other hand, it might also be a case of the producers planing on the show being set in the Showa universe, and changing their mind later on.
Or maybe it's just that the idea of Ultras being famous, well-known heroes was an established part of so many prior continuities that they kept it here, despite it being its own thing. If it was aimed more at kids, then I guess they just wanted to evoke that feel of Ultras being iconic figures that had been part of the culture since before the kids were born, so they created a universe where that was also the case.
Godzilla 2000: Millennium was vague in a similar way to
Cosmos, because it treated Godzilla as an established presence in its world, although it was the only one of the five Millennium-era Godzilla universes that never explicitly mentioned the original 1954 film as part of its backstory (it did so in offscreen material, but not in the movie itself). The lack of specifics bugged me in that case too.
Thought I guess mainly it confuses me because it's only my third Ultraman universe, and in the previous two, Ultras were treated as a new discovery. True, in
Tiga/Dyna, there had been ancient Ultras on Earth, but humans didn't know about them until the start of
Tiga. And in the
Neos direct-to-video series, there was a world of multiple Ultras like in the main timeline, including versions of a couple of Showa Ultra characters, but humanity was encountering them for the first time. So both universes contained "origin stories" for humanity's knowledge of Ultras as well as for the individual Ultra-bonded human heroes. I guess I expected that pattern to continue.