Wanted to leave a note @RokuCarly,
Looks like SlingTV Version 8.133 build 17290 fixed the issues with working on at least my 4670x, (I've not had a chance to test the higher-end 4K Ultra, but I assume it's probably also fixed).
I saw the big update, tried it, and it bounced back and forth between the familar black screen/spinning circle to one with an add for Freeform Free TV channels, and back, and kept doing this for a while, a reboot didn't fix it, so I removed the app, rebooted the Roku again, added the app back, rebooted it once more (out of an abundence of caution) and then tried it, and it finally bounced to a live TV channel and allowed me to sign in! Seems to be working as intended now, amazed how good the quality on DVRd content is!
Now if only their AirTV2 box wasn't a piece of **bleep**, waiting on delivery of an HDHomerun box.
Thanks for everyone's hard work on this, Roku rules!
Endymion