Hollywood is a company town, even more then because there was way less international production. B5 was pitched to every studio long before the first episode of DS9. The executives deciding to greenlight DS9 would know about BS5 even if the showrunners didn’t. Lots of people worked on both shows.

That said, two people have the same idea all the time. B5 really pioneered the idea of a show with a multi season arc that was planned up front. In a time where shows more or less reset continuity after every episode. Shows like Lost pretended they knew where they were going but really made it up as they went along.

Deep Space Nine had a similar multi season arc, which is why I think the poster said soap opera.