In this lecture (15. Petri nets, Basis of The Flow of Tokens), around 5:25, the lecturer uses the word thenfore, I am guessing he is meaning to say therefore but misspeaks. However, he is a native speaker and I'm not, so I just wanted to check if this might be is an old, archaic word form that I just didn't know about.
I watched the video. The instructor just misspoke, he meant "therefore" as you guessed. "Thenfore" is not a word.
Etymologically, therefore = there + for, with the archaic meaning of "there" as "that", which also shows in words like thereat, thereunder. So therefore = for that (reason). Assuming a similar etymological development, "thenfore" would mean "for then", which sounds pretty odd.