Is this position resignable here?

The important question to ask is: "Does Black have any counterplay?" And he has none. The black rooks can't gain an open file, and the black knight can't find a nice central spot to sit on any time soon. Due to these factors, I would think that this position by itself is indeed resignable. And your time advantage gives you an even larger edge. So it is definitely a one-sided situation.

While the position is a very easy white win (effetively two pawns up, no Black counterplay) almost anything is playable at 3|0 time control.

Playing twice as fast as him you got a better position. Maybe he was disheartened.
I hope you see a very easy white plan to win, but sure, usually it's good to keep playing in blitz. It costs you nearly no time and sometimes you get lucky.