From what you mentioned, it seems to me PIN function is tightly tied to Jade wallet, maybe that is the reason why Blockstream Green can't generate seed offline. Maybe it would interfere with Jade wallet functionality.
This is only my wild guess, lets wait to hear from Blockstream...
I seem to be going around in circles with their support. They have confirmed that anyone with physical access to a device and knowledge of the PIN, has the essentials needed to access the Blockstream app wallet and spend from it. So the Blockstream app isn't any bit safer than any other hot software wallet that you know the PIN or password to. The blind oracle doesn't help in this use case.