Only issue i have with that is that surtur is massive AF if it was Surtur's DF the entire castle AT THE VERY LEAST should've been reduced to dust all we got were some marks on a wall
Marks are probably from Harald's attacks.
And Loki probably didnt even utilize the full powers, just incinerated Harald to dust.
Him being massive AF could be a literal plot point if you remember that Rocks said that DF only reaches its full potential if Harald eats it.
Why Harald? Because he is an Ancient Giant. The biggest humanoid in the world.
If a normal person ate it, he would probably turn into a fire giant of the size similar to Loki/Harald at best.
However, if Loki/Harald ate it, they would further increase in size, maybe even capable of wielding the huge sword stuck in Elbaf.