Let me start off by saying that I understand the description given by the referee following the VAR review was all about the ball not being touched by the defender. Interestingly the LOTG say nothing about touching or not touching the ball.
The full text of law 12 as it relates to challenges is
1. Direct free kick
A direct free kick is awarded if a player commits any of the following offences against an opponent in a manner considered by the referee to be careless, reckless or using excessive force:
• charges
• jumps at
• kicks or attempts to kick
• pushes
• strikes or attempts to strike (including head-butt)
• tackles or challenges
• trips or attempts to trip If an offence involves contact, it is penalised by a direct free kick.
• Careless is when a player shows a lack of attention or consideration when making a challenge or acts without precaution. No disciplinary sanction is needed
• Reckless is when a player acts with disregard to the danger to, or consequences for, an opponent and must be cautioned
• Using excessive force is when a player exceeds the necessary use of force and/or endangers the safety of an opponent and must be sent off
A direct free kick is awarded if a player commits any of the following offences:
• a handball offence (except for the goalkeeper within their penalty area)
• holds an opponent
• impedes an opponent with contact
• bites or spits at someone on the team lists or a match official
• throws an object at the ball, an opponent or a match official, or makes contact with the ball with a held object
So, reading through that, it seems that a challenge for the ball where the ball is touched first but is still considered by the referee to "careless" (as defined above) could still be penalised. In fact, I'm sure I've heard or read about such examples.
The other option I can see where a player could (according to LOTG) be penalised even after touching the ball first relates to the words "impedes a player with contact". Arguably the Arzani foul could have been an example of this (notwithstanding the referee's "no contact with the ball" statement). In the Arzani challenge, the defender touched the ball, it did not make any real difference to where the ball was and Arzani could still have reached it, but Arzani was prevented from reaching it because the follow through from the defender after touching the ball contacted Arzani and tripped him. So arguably Arzani was impeded from getting to the ball.
What do the referees out there think, have you had any instruction about how to adjudicate these types of challenges?