The problem is that your opponent didn't claim the diagonal, but instead blocked the pieces in with f3 and e3, and weakened a bunch of squares later on. The double fianchetto is OK.
Double Fianchetto
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Oh yeah, my opponent was illiterate in chess. But I think stockfish thought the double fianchetto was bad because 4. Bg2 Nc6 5. Bxc6 and doubled pawns for me. But then that would leave a hole in the white kingside.
I doubt it - the fianchettoed bishop is worth much more than the knight on c6, and after you push ...c5, you doubled pawns are not an issue, since they don't interfere with your pieces (in fact it frees your queen), and you can fianchetto anyways.
If stockfish thinks it's "bad", it's certainly slight and not that significant.
When he did it, and when I did it, I thought the double fianchetto was bad. The kingside fianchettoed bishop would be nice and active. But when the king bishop is pointing towards the queenside I would want the queen bishop pointing at the queenside as well. I think Bf5 would be better than b3 and the fianchetto.