All the damage rocks took before the final attack was not heal.
We did not see any regen or was it mention that rocks heal.
What was said was he was using his CoC to cancel out the attacks done by that garp and roger .
So why would he need to heal when he has ben cancelling out the damage ?
That is why they said they had to do one big attack because rocks was not getting damage .
The fight was loki giving him 4 hits and he healing from them .
If we going with what Gaban said it was not strong damage since Harald heal fast .
Then harald allow him to kill him .
Read over chapter 1165 rocks had no damage to heal because he was cancelling out roger and garp attack with his CoC so there was no damage.
Rocks wasn't cancelling out the attacks: he was talking them and still regen them. Obv. not all of them: he manage to stop some of them, as Harald.
Roger and Garp couldn't go
beyond his defence and deal actual long standing damange to him.
On the other hand, as said, 17 YO Loki, in 1vs1, was able to make Harald return to his sences and wasn't even that damaged.
After the got the fruit, the fight was quite one sided.
But I say even more: id It Is as you say, Loki would be even more incredibile since Roger and Garp couldn't go beyond Rocks defence while Loki did more than once.