What happened? The guy who made the last mistake lost
You kept a bad hand, and then topdecked like mad. He kept a bad hand and didn't. Just look at your sig. Everything looks nice when opponent does nothing four turns in a row (copyright rstnme). And he got unlucky in that you were able to kill his Nautilus twice very quickly.
As for your actual mistakes:
That hand was a mulligan. It worked out this time, and there's no telling how good a six-card hand would be, but you had no critters to play for the first four turns, and no lands to play your five-drops with.
Armament on Siren was OK. You needed the mana later on for other things. I think he would have bestowed Nautilus on Siren turn five, possibly when you were tapped out from playing Ornitarch, and that would have hurt. No way you could have known that, but it was clear you will finally be playing things on turn five.
Killing the Artisan was OK.
Putting the counter on Ornitarch was a mistake. But I had to learn this the hard way too.
When he attacked for the last time, you should do nothing. He can't target Nautilus with anything anyway. Just keep the Glimpse for any blockers he might play post combat. After that attack he's dead anyway, unless he has something really good, and you should just wait to see what it is. You have lethal on board and a bomb in hand.
He might have had a tempo play in hand which might make it possible for him to race, if you didn't kill the Nautilus. And he conceded because you killed it.
Kinda nasty of me, but you wanted to know. Nice to see you winning drafts.