A number of us have, from the very beginning, objected to 135 format 24 x 36mm being used as 'full frame' definition, as if the 135 format were the standard against which all formats were judged...we brought up the point that 24 x 36mm was actually referred to as the 'miniature format' (apparently somewhat derogatorily) in the 1950s and early 1960s.
But for the common usage of 'FF' we all had to give up the fight, as the industry and journalists all referred to the 'crop' vs. 'FF' bodies both using the same lenses.
In the medium format world, the square 6x6 format was marketed by Hasselblad for a long time as superior to the 645 format. That marketing positioning disappeared as soon as Hasselblad started to put digital backs on their bodies, and all of the sensors were LESS THAN 645 (43 x 56mm) in size. For example one back offered by Hasselblad was 32.9mm x 43.8mm or a 1/1.3 crop vs. the 'FF 645'. It has taken medium format many, many years even to EQUAL the 43 x 56mm dimension. Ergo, in the medium format world sensors smaller than 43 x 56 were 'crop' and those achieving 43 x 56 (or thereabouts) are 'full frame'...and even 40.4mm x 53.7 is not truly 'Full Frame 645', but we'll let 7% sneak by.
That brings us back to 'FF' being a contextual reference, and NOT establishing 24 x 36mm 135 format as 'the standard reference'.