He probably played the old tpt when it was out, and then saw how the server was shut down so he made a new one and started developing the game once Skylark stopped. I don't know if the old version was open source, I think he went to Skylark to get the code for the game, and started developing it from there. He didn't know Skylark in real life though.
I didn't even know about tpt back then, so I can't say for sure. Someone more active at that time would probably know exactly how it happened.