Absorbing the spirit bomb/energy has shown in the past to be a huge power boost for a short time. Trunks killing Goku Black with his sword and the Super Dragon Fist we've seen in the past. Think the spirit bomb absorbing only kept Goku alive though since his ki was completely depleted, and the spirit bomb energy became a source for him. I agree tho that it still doesn't explain why Goku got that powerful tho, unless his body becoming unhinged unlocks more power on some pure adrenaline/Limitless NZT brainpower shyt.