Thats just arguing semantics at this point. So let's argue semantics properly.
Fanatio's HPB did not "melt" the cathedral objects. It literally made them explode. What was left "molten" was literally the hole left by the explosion that is in the very previous sentence.
"Melting" and "Burning down" are the same thing. One is slower and requires a change of state, that's it.
It wasn't the kind of stone that would melt from a simple fire
So stones do melt, not "evaporate". But as shown in previous sentences, they literally explode due to the heat the Heaven Piercing Blade inflicts upon them, as indicated by the literal "explosion" that happened upon contact.
Which is the next point in this semantics. Stone is not end all be all against fire. It doesn't "burn" the same way wood does. It still is affected by extreme heat, cracking, crumbling, exploding.
Also, Imperial Palace's are not pure marble buildings like the Central Cathedral. They are not log houses either, but they are normal buildings. If "stone doesn't burn" was an excuse, fire would be a lot less of a hazard even in our real lives, especially in Mediterranean regions where houses are made of stone due to acting as good isolation against hot summer days. Stone structures burn just fine due to everything else involved in their construction and decoration.
Factor in Fanatio's HPB causing massive explosions on the facades it makes contact with, because the stone cannot take the heat, you got yourself your huge pile of rubble that takes 3 months to clear up.
A fire wouldn't be able to cause stone to collapse like that. Ergo, it has to be from the Memory Release of the sword.
I dont understand, are you claiming it evaporated, or are you claiming the Heaven Piercing Blade just manually and carefully deconstructed the estate?