>That's completely wrong.
No, it's completely correct.
The production companies were The Douglas S. Cramer Co., Bruce Lansbury Productions, Ltd., Warner Bros. Television and DC Comics. That's a four-way split even if two of those are owned by Warner.
Bruce Lansbury was a producer on some of the most popular shows of the 60s/70s (of which Wonder Woman was never one, because it wasn't popular). He died in 2017 and I honestly don't know what happened with his production company, but he held rights in Wonder Woman because his company was credited on it. He was by no means a Warner Bros.-only producer.
Douglas S. Cramer is still alive: he even provides commentary on the season 1 DVD of Wonder Woman from 2007, which indicates that he had to be involved with that release.
In addition to that, the show was produced by Warner Bros. Television et al, but it was produced *for* as you say ABC (part of the Disney group today) and CBS - the first home release, on VHS, was explicitly in partnership with Columbia House, part of the CBS group, indicating that at least in the early part of this century CBS retained rights in the show.
>the actual small companies producing it
Yeah, so the reason these small companies exist is either to bring in technical expertise (manage my show for me) or finance (invest in my show for me). The former doesn't always result in part-ownership of the rights in the finished production (not the rights to make more of the same in a different show, which is something else entirely). The latter always results in part-ownership - it's an investment, after all. You want me to guess, Cramer was the moneyman, Lansbury was the technical guy. But maybe both invested: it's not uncommon.
>it's on DC Universe
Yeah, but that only means WBTV has the right to broadcast it on their service. Which is not uncommon even for disputed control over a show - look how long Batman 66 was in reruns after all - and definitely is not proof of full ownership.