It's a possibility, also, assuming the TNG books are wrong. It's never mentioned as such, however, and with Voyager saying Transwarp is something the Federation don't have, I think it's unlikely.
Excelsior's "new" warp drive would be so called because unlike the old engines it would be theoretically capable of entering transwarp, in much the same way that a jet aircraft -- unlike a propeller driven aircraft -- would be capable of breaking the sound barrier.
By the same token, it's possible for prop-driven aircraft to reach supersonic speeds if they dive from a high enough altitude under ideal conditions. It seems to also be the case that a starship with a conventional warp drive could enter transwarp in the presence of, say, a Xindi subspace vortex or by creating a quantum slipstream in front of them (or using the Borg transwarp conduits which operate the same as both technologies). True transwarp, however, implies the ability to enter these deep coherent "tunnels" in subspace by brute force alone, without relying (too much) on technobabble trickery or prearranged conduit networks.
That would nicely explain why the Xindi "subspace vortex" thing was never used again in the 23rd or 24th centuries; the Xindi are instinct in the prime timeline and nobody's ever heard of them OR their vortex trick. Starfleet in the Abramsverse has Jonathan Archer's logs to work with, so they probably figured out a way to generate those vortexes artificially using a severely overclocked warp drive.