Not sure what your basing your reply on but in my house it's not true. I attached images of two main basses... The single cut has access to the F (on the G string), while double cut has access to an A (on the G string). The 4 fret difference is important to me.
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In this case, the improved upper fret access on the SC is most likely because it is a neck-through-body instrument.
The Roscoe is a bolt-on, which requires more "meat" of body wood around the neck joint to support the neck heel.
Even with that - the shape/depth of the treble cutaway is not dictated by the presence or absence of an upper cutaway.
It is simply a function of the design and layout of the instrument.
A good counterexample is the Les Paul (single cutaway) vs. SG (double cutaway) - the SG has WAY better upper fret access,
because the guitar is designed for it.
Take this thought experiment: if your Roscoe were an unlined fretless, you could move the bridge a few inches towards the neck.
You would then have a full two-octave range. Ditto in reverse for the Fodera - what is important is the *relationship* between the string scale
and the treble side cutaway.
The modern SC shape originated by Fodera was intended to stiffen the neck by shortening the cantilevered portion.
The goal was to improve sustain, stability, and frequency response. The greater stiffness is much more relevant than the
oft-cited increase in "neck-to-body contact", which is a dubious variable anyway with an instrument like a Fodera where it's all glued together.
Neck stiffness is a HUGE determinant in tone/response/stability etc.
Fodera should get due credit for going out on a limb with this design way back when - they have moved the field forward, and a lot of guys
are building SCs now, including myself (though mine are bolt-ons...different conversation).