ubiquitin,
I skimmed through that thread, and if anything it strengthens my question to you. Not a single poster in the thread claimed that the Avos literally kept every single one of the 613 mitzvos, i.e., giving terumos to a kohen and then eating them in a pure place as a kohen.
The strongest assertion in there was by dafyomi2711, that the Avos had the same Chumash, Mishna, and Gemara that we have, and therefore, they would do the mitzvos exactly how we do them today. He was quickly stymied by questions from on the ball, however, and subsequently accepted that the Avos may have had a different perception of the Torah than we do today, and even followed mitzvos differently.
I think the source of your bother is a misconception.