On the other hand, the fact that they did a Supergirl series rather than a Superman series seems to imply that they didn't have the option of doing the latter. Not that I agree with the sentiment, but most people in Hollywood would probably see Supergirl as a less desirable character for adaptation than Superman. So even if they have permission to reference Superman to an extent, there may be a preference to leave Superman himself mainly to the movies.
Perhaps, although from all the interviews I've read I get the impression everybody involved (including CBS) was
genuinely interested in making a Supergirl series, and not a Superman one. And not just because it was the only thing available to them.
In fact I think it was even said one of the head executives got a bit teary when the idea was being pitched to them, because the character meant so much to them as a young girl. And it's also easy to see why Berlanti would far prefer the challenge of doing a Supergirl series over a Superman one as well, since Supes has already been interpreted for the screen multiple times.