Hey.

https://musicbrainz.org/doc/Release/Format lists HDCD as a format like enhanced CD or CD-R, however it isn't (even though it's perhaps been marked as such)

In fact it's just a form of audio encoding that fits in normal PCM audio, and thus can be part of any CDDA, i.e. a enhanced CD can have HDCD content as well.

When you rip a HDCD, the PCM audio file will be HDCD as well, and there are tools which one can use to decode this.

Further it should be noted, that many HDCDs marked as such aren't actually HDCDs,... even those where some basic HDCD markers are present in the audio signal, it still doesn't mean that any HDCD encoding feature is actually used (this is a well known issue, as apparently some mastering hardware simply added the base markers even though no HDCD techniques were actually enabled).
Moreover, AFAIK, HDCD isn't a property that applies to the "whole" CD but it was possible to enable certain features at least per track (maybe even more granular).

So it seems to me, that the way MB tags HDCD is not correct.

Cheers,
Chris.