[SBazB]

The promo videos were shot on film, so if the original film footage still exists, it can be rescanned in HD quality.

There are two scenarios here: a) they didn't have the budget to go back and rescan the film footage, and re-edit all the correct shots together, or b) the original film footage no longer exists. In which case, they would have to use the best quality version available which is probably the final edited version copied to a 1990s digital video format, and therefore standard definition only, meaning it has to be upscaled (basically inventing new lines of information in the picture, based on the existing lines) to get it up to the technical specs of HD.

It's all just semantics (and a marketing ploy), but it irks me that the new uploads are presented as "HD", when the source footage probably isn't.