I support this claim. Especially given the fact that the 3 OAVs that form the Megazone series, tell a single story (cut in three) and were produced within a few years have each their own page.

Also, there are weird choices in the Gunbuster informations: for example, some other multiple episodes OAVs are registered as "scripted show" while the Gunbuster series are classified as "video". (I also noticed the lack of a "creator", and other things hidden behind lock.)

  Diebuster is the sequel series

TMDb groups sequels anime series on the same entry: https://www.themoviedb.org/bible/tv/59f743289251416e71000037#5b71c6b4c3a3684161000001

some other multiple episodes OAVs are registered as "scripted show" while the Gunbuster series are classified as "video".

The video type was literally created for OVAs: https://www.themoviedb.org/bible/tv/59f73eb49251416e7100001f#59f7403f9251416e7100002a

I also noticed the lack of a "creator", and other things hidden behind lock.

Yes, https://www.themoviedb.org/bible/tv#59f7403f9251416e7100002c