DY
“Who? It’s stated in Chazal. Some meforshim say not in the precise form as after matan Torah, but they don’t argue with Chazal.”
Anybody.
Of course they didnt literally keep the entire Torah.
sippur Yitzias mitzrayim?
Michiyas amalek?
writing a sefer Torah?
building a mishkan/beis hamikdash?
appoitning a melech
Giving a get?
Yibum and chalitza?
Ben soreh umoreh?
doing the Avoda
Giving Teruma/maasros?
Eating teruma bethara?
etc etc
Yes Im sure there are vertelach about some of the above. And soem might be easier to answer than others
Nobody can literally keep the entire Torah some mitzvahs are exclusivly for Kohanim some exclusively for non-kohanim.
All the more so the avos, with no sanhedrin, beis hamikdash, kohanim leviim etc..