FF3 DS supposedly doesn't make much use of the DS's unique features.
And it's "better than PSone" graphics were probably somewhat expensive to produce, and since they're polygon-based they could very well look better if ported to the PS2.
And while the original version apparently can't be ported, nor can it be the foundation of a remake, it can pretty easily be emulated and tossed in for free with a full remake.
And the latest FMV shots are in a 4:3 aspect ratio, at just about 640x480 resolution (600x450), which is totally wrong for the DS, but just right for a home console.
I figure if the FMV shots aren't direct confirmation of a PS2 port, they're at least a giveaway that Square's building the game to be the definitive version of FF3, to be used now and in the future, and not just a custom-tailored DS-exclusive game.