If they're offering the premium cabin as a "good deal" buy-up offer on the app, chances are the cabin is wide open enough that your upgrade might eventually clear. The folks at RM are just trying to monetize the seats as much as possible before releasing them for upgrades, since upgrades bring in very little or zero additional revenue. (I do see how it's bizarre to be on the upgrade waitlist for miles+$ copay and then see an upgrade offer popping up in the app for less than that. But those are two different programs and ways of getting into J....completely separate from each other.)
It's just the name of the game these days. As noted above, if it's that important to you to fly in J, just pay for J.