ROJAN: No form of transmission can penetrate the barrier.
It's pretty clear that he's talking about any sort of communication not just FTL.
Oh, don't take every little syllable so literally. It's a work of fiction. There are a lot of things in
Star Trek that don't make sense, and it's less frustrating if you let yourself be mentally flexible enough to rationalize them in ways that make more sense.
Heck, people speak figuratively or inaccurately all the time. Real speech is fraught with error and imprecision, so it doesn't make sense to assume that every single word ever spoken by characters in a TV show is absolute gospel. And sometimes people use shorthand. Rojan could have meant "no form of FTL transmission" but felt it was so obvious that it had to be FTL that he saw no need to be that specific. And I think you're forgetting an episode or two of TOS (and TMP) where Kirk was surprised to hear that a transmission was in "old-style radio," as if they considered that an obsolete technology in their era. Rojan could have the same mentality, thinking of subspace as the default form of transmission and neglecting the existence of a more antiquated form of communication that it wouldn't occur to him to use. (If you say you're going to send someone a text message, do you find it necessary to specify that you're doing it by cell phone rather than telegram? Of course not.)
Then there's this:
SPOCK: A robot ship could be sent to Kelva with the Federation proposal.
If it's just the barrier blocking communication, why send a ship all the way to Kelva with the proposal?
That's easy -- because it's 2.6 million light-years away! Subspace radio doesn't have unlimited range or speed, otherwise
Voyager would've been in constant communication with Starfleet from day one. And that's across distances only 1-2 percent of the distance to the Andromeda Galaxy. Obviously a subspace signal just couldn't reach that far.