The Viltrox EF-FX2 is a speedbooster, therefore it contains optical elements that turn the Canon 85mm 1.8 into a 90mm f1.3 lens.
Just a small correction - it turns Canon 85mm f/1.8 into a
60mm f/1.3 lens (or crop sensor 90mm
f/1.9 full-frame focal length / depth of field equivalent).
Just a small correction to your small correction, it doesn't turn the 85mm into 60mm in turn of field of view but to a
90mm (85mm X 1.5 X 0.71) just as Marius said. In case you wondered, I've actually put side by side with my cousin's Sony a6500 + Sigma 56mm, and my Viltrox speedbooster + canon 85mm being narrower in field of view.
It does turn (physical) 85mm lens into (physical) 60mm lens (in regards to field of view). Only then, on a crop sensor, that translates to 90mm full-frame equivalent lens field of view - but there (physical) 85mm translates to 128mm equivalent to begin with (so it`s not that 85mm became 90mm).
And my sentence you quoted says as much (note "crop sensor
90mm f/1.9 full-frame focal length / depth of field equivalent"), so not sure what you are correcting here...? (unless "focal length equivalent " part wasn`t clear enough, meaning "field of view equivalent ", too, indeed)
The depth of field (for viltrox + canon 85mm combo on Fuji camera) of fullframe equivalent is indeed f1.9 like you said, but the light gathering capability did go up to f1.3 as Marius said. Why F1.3? Because the Speedbooster bend the light coming thro the lens to focus on a smaller area of the apsc sensor which resulted in higher concentration of light. In effect, you gained approximately 1 stop of light.
And I never said anything different - being _exactly_ why I made it clear what f/1.9 is equivalent of

Lens` light gathering ability is... well... "speed boosted", that`s where the name comes from

(also known as "focal reducer", due to what it does to focal length = field of view).
Dont forget the fact that F stop affect both the depth of field as well as the light gathering capability.
I think you missed the point of my reply, merely being that it doesn`t make 85mm f/1.8 lens (which are lens`
physical attributes) into a 90mm (which is boosted
equivalent focal length)
and f/1.3 (which is boosted
physical light gathering ability) at the same time.
Physically, boosted 85/1.8 becomes 60/1.3, period. Field of view equivalency involved, boosted 128/1.8 becomes 90/1.3 (where light gathering gain is visible). Depth of field equivalency involved, too, boosted 128/2.7 becomes 90/1.9.