That's right. There are also essentially 2 different kinds of FMV on the SegaCD: one where the video is made of rapidly decompressed pictures, like a normal codec, and another where everything is really just made up of dynamically loaded huge sprites and background planes. Sonic CD is a good example of the former, and Popful Mail of the latter. I think Lunar 2 uses a mix of both.
Some games may share the same codec. I think Cinepak created the one used in most FMV games. AFAIK nobody has done anything to enable a PC to view any of them, though.