The way to defuse your potted pal is as follows:
1) DS9 is a direct spinoff of TNG, which is owned by Paramount.
No one could pitch a ST spinoff elsewhere.
2) DS9 was created by Berman/Pillar, who were under contract to
Paramount working on TNG, and thus would not have pitched it to Warner
Bros. at any time.
3) B5 dates back to 1987. If he would like to see some sneaky
references to it in print, tell him to go check out a copy of OtherSyde,
my second novel, published in around 1990, which contains a reference to
Babylon 5. Also, if he has a bunch of old Starlogs, if he wants to check
in around 1988, he will find an interview with me in which one of the
things mentioned is that I'm working to sell a show called Babylon 5. A
final reference to B5 is in an episode of Captain Power called "Final
Stand," which I slipped in for funsies.
Your potted (to use your phrase) pal's obstinate opinions are
irrelevant; the facts, in hard print, speak for themselves. He's got
things in reverse; B5 was pitched to Paramount in 1989; DS9 was never
pitched to Warners.