Exactly this. Also my experience and FYI, I genuinely think WiFi Call was never supported on OG Pixel as I had it and was able to get VoLTE to work (after calling to get IMEI Whitelisted) but the WiFi Call toggle was grayed out and always had been in the #4636# menu which leads me to believe that for this model, WiFi calling was just never supported at all by Rogers (i.e. WiFi call won't ever get provisioned if phone model = Pixel). For Pixel 3, both VoLTE and WiFi Call were natively "available" in the #4636# menu, but wouldn't work until IMS registration worked (which as you pointed out seem to only work for non Rogers device when someone on Rogers' side "adds" your IMEI or "toggles" provisioning to your phone). On my Pixel 3 for instance "Video calling" is grayed out which leads me to believe that at the moment (or maybe ever) ; carrier Video calling (Through the Phone app) will never work on any Pixel 3 on Rogers, unless this toggle becomes available eventually.
It's really a weird concept and i genuinely don't get what companies get by doing this. An option to self whitelist your IMEI for instance in the customer space online would be so easy (just like you can update your SIM card number if you swap SIMs). After that, if your phone's VoLTE, WiFi calling (etc.) doesn't work and it's a non Rogers device, I would understand that Rogers will do what they can to help but will be limited in what they can do as they can't cover all phone models / versions. But for phones like Pixels, Essential Phone and the "big brands" (Galaxy phones for instance) there is no reason why functions like VoLTE and WiFi calling couldn't work out of the box really. Of all phones ; ESPECIALLY on the Pixels it doesn't make any sense at all.