The HHGTTG example there is from 2005. This is the old undocumented way it has been done for decades. It fits the release groups that this release is in with the CD releases of the same item.
These are not “bootleg”. These are official radio broadcasts. These are categorised in the Radio Drama type and fit in perfectly next to the CDs that are later released of them.
They arrive in the database as people take copies of these radio dramas from the radio and want to know more about them.
They are broadcast as one offs. The show over six episodes. This is what the BBC has done for decades
Release group “Doctor Who: Shada” by Douglas Adams starring Paul McGann, Lalla Ward & John Leeson - MusicBrainz
They are like audio books, but with more actors. (We have already talked a bit too long on this one 
)
Older threads on the topic:
Radio Plays (Audiobooks or Spoken Word)?
Album vs Broadcast
The better guideline to look at is the audiobook one. Style / Specific types of releases / Audiobook - MusicBrainz
Until only a few years ago, when the Audio Drama type was created, these shows would appear under audiobook category. Which is why they more closely follow this guideline.