This is puzzling to me. For what reason would one want the original Xbox SC 2 when the objectively superior HD online version is already available via BC? Also, Halo 1 and 2? Couldn't you just play them through the Master Chief collection and just switch to the retro graphics? And Fable 1? Wouldn't it be less effort and make more sense to simply make the Anniversary edition available via BC? And both Ninja Gaiden and NG Black? Isn't this kind of a wasted effort as Black is the superior version? And MGS2....same situation, why not just make the HD Collection BC?
As I said, it's just a bit puzzling.
Soul Calibur 2 wouldn't have much use beyond the fact that someone possibly already owns the OG Xbox version, and so wouldn't be forced to buy another version to play it again. This reasoning honestly should be extended to the entire library (so lump MGS2 in here as well)... just because there's a better version existing, doesn't mean someone should be forced to rebuy it. That line of thinking is how PS2 on PS4 situations happen. We've kinda been over this point numerous times regarding 360 games becoming BC when there's already an XB1 version.
Halo CE's retro graphics are actually notably different from the original game's, and not generally in a good way. If someone did want to play Halo CE looking how it used to, I would imagine they'd prefer the OG Xbox version, even at the expense of resolution and framerate.
Fable 1? Dunno. We have no idea how much effort any of this takes. I can imagine that if their OG Xbox emulator is sufficiently evolved, then near any OG Xbox game is probably easier than the average Xbox 360 game, simply due to the performance deltas between each console and the XB1. OG Xbox was also already an x86-based console like the XB1, as opposed to PowerPC based. Still though, Fable Anniversary still not being BC yet (or even being released only on 360 in the first place) doesn't make a lot of sense.
Ninja Gaiden Black is basically a GOTY edition of Ninja Gaiden. If one works the other is likely to work at exactly the same time. There's likely zero effort involved to get one running over the other.