The Spirit Bomb did explode... so unless it acted as a type of Nuclear bomb and his body was radiated from the outside to in, then he did not absorb it. I think Goku was under the rubble/dust. That's my opinion is he was radiated with energy like a microwave from the explosion.

Ultra Instinct is not a ground breaking new form which I find boring if every nub can obtain it. All it is access to a new type of power or a new way to use the power. Not so much a power up. I guess if they intend to retcon, better to do it now from here on we all know what's going to happen if every person can use Ultra Instinct like Half Human Saiyan hybrids can use SSJ.