There has been 3rd party made decoders for OMAP that can handle bigger resolutions/profiles than the ones provided by TI. I guess Nokia hasn't dished the money to buy them and are using the TI ones.
Odd thing is that I read that High profile is about as slow to decode as Main profile (both use CABAC entropy encoding) so it's odd if N9 supports Main profile but not High? And I'm not really worried if it can't decode 720p, because the screen can't show it without resize anyway.