+2 on the Wiki article, which highlights regularities as well as inconsistencies in nomenclature.
"Bass" and "contrabass" tend to be relative terms, applied to larger and lower-ranged versions of the corresponding "prime" members of the relevant family. This alone dictates the fact that similarly-prefixed instruments have different ranges across families: a bass clarinet has the range of a baritone sax, not of the bass one.
Moreover, whether the prefixes mean "one octave lower" or just "lower" apparently depends on the whims of whoever named them first. For example, the bass flute is pitched lower, and one-octave so, than the standard flute, and the contrabass flute one further octave than the former; the same happens with clarinets; it notably does not happen with saxes: the bass member is one octave below the tenor, but the contrabass is one octave below the baritone and two below the alto, meaning the contra and the bass are just a fifth apart, not an octave. Finally, within the bowed string (fretless) family something intermediate happens, with the bass member i.e. the cello going one octave below the viola (which is at least linguistically the prime member: it gives most of the others their name, not accounting for shortened, common forms), but the double bass being able to do the same, relative to the cello, only with a C extension (or having an extra string).
In the case of the Niibori classical contrabass guitar, as I said, the "bass guitar" to which the former is "contra" (meaning "lower", albeit not "8va-lower") is not, I repeat, not a Fender-tuned (or upright-tuned) E1-A1-D2-G2 instrument, but a ~27.5" scale guitar tuned B1-E2-A2-D3-F#3-B3, like an electric (or acoustic steel-string) baritone guitar:
Contra guitar / low octave guitar
[Invalid or Expired Link Removed]
Contra guitar / low octave guitar
Invalid Link Removed
In other words this other, second-lowest of Niibori ensemble guitars called "bass" is, in turn, pitched only a fourth below the "prime" Spanish guitar (which was probably perceived as being already a bari-tenor instrument in itself).
Again, I suspect that Hiroki-sama had in mind the same, more "absolute" if you will, meaning of registers that @Garret Graves mentioned; or, as I surmised above, he planned to have some/all instruments in his orchestra (roughly) correspond in range to a bowed counterpart.
Also of note, the first recorded mention (in the 1899 catalog of Chicago-based luthier Joseph Bohmann) of "contra bass guitar" as mentioned in the relevant Wiki (ultimately
here) is of an instrument we know next to nothing of, and which may have been tuned lower than E1 but it's reasonable to think it wasn't.
^Here too the naming was probably to be intended as: "contra bass", that is to say as low as (but likely not lower than) a "contrabass"=double bass is. Then again, in 1899 there was no instrument named "bass guitar" whose name could have been a source of confusion: the Regal Basso-Guitar appeared in 1936 I think.
[Final EDIT, I promise -
Even the "tenor guitar" owes its name to a similar phenomenon of interference from other families: in its traditional tuning, C3-G3-D4-A4, it's simply the guitar version of the tenor banjo, which in turn may be thought of as a banjo version of the mandola, with the same tuning of the latter but a range-name perhaps more appropriate for the octave mandolin, with its G2-D3-A3-E4 tuning. Speaking of which (and in light of the obvious dependence of the mandolin family from the violin one, re names and uses), why does a bowed G2-E4 instrument not exist to bridge the gap between the cello and the viola? Apparently
it did at one time, but it fell out of favour in the classical period proper.]
All the above being said, @ContraGuitar is by no means alone in his opinion. It was expressed in similar terms by
@Cristofre in this thread of his:
Invalid Link Removed
"Bass guitar", as we commonly intend it, exists and has for decades deeply shaping a number of music genres in the process (notably the most popular ones), and the objection to another instrument being called in English "contrabass guitar" even though it's pitched no lower than the bass is absolutely sound.