In Old English, ⟨ð⟩ (called ðæt) was used interchangeably with ⟨þ⟩ to represent the Old English dental fricative phoneme /θ/ or its allophone /ð/, which exist in modern English phonology as the voiceless and voiced dental fricatives both now spelled ⟨th⟩.
Except it’s not thorn. Thorn is Þ.
You are correct. In my defence:
Oh ha. Looks like you looked it up as I was looking it up.
Still, whatever it is, doesn’t really answer the original question which was about why the user above was doing it.