I'd agree it was an algorithmic bug and not an implementation bug.

From my reading, it essentially got AlphaGo into a state where it was no longer reading the board correctly. The algorithmic bug was play a decent but extremely improbable move and AlphaGo won't know how to respond.

Or by a similar argument, I think most people would say that if Lee Sedol hadn't played that move and the game continued 'normally' he would've lost like the other games. The rarity of the move is why he won, not the "strength".

Essentially they trained the app on too specific data. Their main fix was to retrain the next version from "scratch" instead of from moves humans are likely to make.

If I remember correctly, the problem wasn't that it didn't see the move, the problem was that in response to the move it played a few really bad exchanges, like the stupid-looking wedge in the bottom-left, and adding stones to a dead group on the right. Playing bad exchanges when behind is a bad idea even from AlphaGo's perspective. It's still not correct to call it a bug, though.