Sadly sideloading xbox fire tv app on a google tv doesn't work :(
You can sideload the xbox fire tv app on a android/google tv. However you are unable to sign in as it boots you to the wrong page. On a fire tv its supposed to have you do some steps.
On a side note you can sideload the new xbox app to an unsupported fire stick and sign in like normal.
Yup... that's the sad part that i can't argue with
Sometimes i wonder why some Android devices has very tight security in bootloader ? Android is just linux with fancy UI & linux is meant to be freedom to do whatever you want since you own that device
Tbh...in the end Android feels like what windows did....no freedom or whatsoever