I think the issue here is that Apple made it SEEM as though you just place your finger over the button, the ring detects your finger from the electricity in your body and activates the scan. This method would not result in unlocking the phone in your pocket as even if the ring detected skin there would be no resulting fingerprint to unlock the phone.
As far as I can tell this was the original way apple intended the feature to work, however the ring itself was using more power than was favourable so they required the button to be pushed first, which activates the ring, which then takes a fingerprint scan.
As far as I can tell this was the original way apple intended the feature to work, however the ring itself was using more power than was favourable so they required the button to be pushed first, which activates the ring, which then takes a fingerprint scan.