Yeah you are probably right, and this lack of a need for a connection to the films is also evident when you see Rebels in its entirety. Spoilers: Rebels and Rogue OneThere is never a really huge crossover event linking Rebels to Rogue One. Just a few cosmetic links like Saw Guererra, U-Wings, name dropping Stardust/Death Star, Death Troopers, etc.
At the time Rebels was airing it seemed like such a safe assumption. The end of Rebels season 4 should be almost to the point that Rogue One happens by a few months or so.
I don't think it's that they couldn't link to Rogue One (since that story was already complete by that point, so they didn't have to worry about spoiling or contradicting anything). It's more that they decided not to, because bringing their own story to its natural climax was more important than playing connect-the-dots. That, plus the fact that the character and story arcs they'd introduced needed to be wrapped up before Rogue One/A New Hope to explain why they weren't factors during the OT.