They even mentioned it’s “immune to magic” like one. Maybe that was the inspiration?
Well... The original (FF/2nd ed.) Tarrasque wasn't "immune to magic", really..
It was simply so damn huge with such a thick skin that weapons and magic wielded by/available to Adventurers simply didn't have any impact on it.
Neither did siege weapons..
You'd have needed a full wing of dragons doing a focussed attack to even scorch it/get its attention ( if you could possibly get the dragons to agree on such a harebrained idea regarding "attracting its attention" to begin with..).
But an attack like that would crater and glass anything it hit to begin with..