I’ll say right off the top, SIGMA leaks have always been relatively unreliable. I don’t post a lot of SIGMA related rumors because of reliability concerns.
I have been told by more than one unknown source, that SIGMA is readying an announcement that will address their plans for the RF mount.
One of the sources claims that the Canon EOS R5 has “accelerated” SIGMA’s desire to get RF mount lenses to market sometime in 2021.
The same source did say converting current ART series lenses to the RF wasn’t in the cards, and that SIGMA will have a “unique lineup” for the RF mount. I suspect there will be some overlap of focal lengths between the E mount and RF mount lenses.
As always with anything [CR1], please take this with a grain of salt.