How did Andrea Mengucci receive a game loss for incorrectly registering his card on arena?
The player (Andrea Mengucci) wrote a wrong card to his decklist after the draft. This constitutes as a decklist problem. The fact that the tournament is partly played on MTG Arena, doesn't matter here. Appropriate penalty in this is case is a game loss.

Magic IPG (Infraction Procedure Guide) states (https://blogs.magicjudges.org/rules/ipg3-4/):


  3.4 Tournament Error – Decklist Problem
  
  Definition
  
  The decklist is illegal, doesn’t match what the player intended to play, or needs to be modified due to card loss over the course of the tournament.
  
  Penalty
  
  Game loss


The player is fully responsible to mark the correct cards to his deck registration sheet. They aren't checked ahead of time.

One could argue that penalty isn't fair, but it's a correct call in this case, according to the rules. Maybe this motivates discussion about should the rules be changed.