They likely locked the R3's specifications in early 2019, but just designing a new name plate that says "R3" and replacing names in firmware, user manuals, and product packaging is a much easier lift. The decision of what model name any camera gets can be left fairly late into the product design cycle.
In fact I just scrolled through all the menus in the R3 and the only mention of "R3" is seems to be the default Wi-Fi network the camera can create contains the string "EOSR3" in it. The only indication that the camera is branded an R3 externally is the name plate below the EOS logo. Even the QR code that you can scan from the menus to get the online product manual doesn't say R3 in the URL (it goes to
https://cam.start.canon/en/C010/).
Not saying Canon did it, but changing the branding is not nearly as big of a deal as producing multiple sensor options into EVT stage.