It is, it's just that "th" encompasses many sounds. The th in "the" is þ, while the th in "father" is closer to ð. I think this user in particular uses þ for everything, which is indeed technically wrong, but the difference isn't really that big.

Isn't it, though?? Why do you say it isn't? And I swear to fucking god if you respond "because it isn't"... Smartass lemmings.

That is how the above poster use it and the answer to the above question.

https://en.wikipedia.org/wiki/Thorn_(letter)