Yes, that's the common story. The counter to it is that Paramount already had ideas for DS9 in motion at the time JMS came to them. It isn't like, "set a space show on a station" is an incredibly difficult idea to come up with. I don't think we are ever going to know the real truth of the matter, and I don't think the speculation is of any value to viewers.
True enough. Hollywoood seems to have a common pattern of 2 movies of the same general concept coming out at the same time.
This year has had 2 White House invasion films. The End and The World's End are both post apocalyptic films. The Illusionist and The Prestige were both films about magicians (coming out around the same time). Pretty much every year, 2 different studios are making variations of the same idea (Dredd and The Tower).
What proves DS9 wasn't a deliberate clone to me was that the fiest 3 seasons were episodic where B5 was always serial. Then DS9 switched to being serial (and the show got better by most people's opinion). That switch is when DS9 raised its head, looked at what B5 was doing, and copied it. Until that point, DS9 was being made in its own little idea bubble.
B5's serialness wasn't an original idea. JMS has said in interviews, that he wanted to emulate the serial story telling that BBC shows did (I don't recall him naming anything, merely that it was an inspiration that series have a story arc to tell and then end).
What B5's contribution to American television was that nearly every TV show is now serial. Events from a previous episode carry over to the next. There's no great reset to status quo ending of every episode that lets you watch them in random order like we had in the 80's.