The attacker is in an offside position when being fouled by the defender. But as the attacker is not interfering with play and is not interfering with the opponent then awarding the penalty kick was the correct decision. This is also the official statement by UEFA.
The attacker is not challenging the defender for the ball as the ball is clearly not going anywhere near them. Therefore the offside offence cannot occur.
Being in an offside position is not an infringement itself, but a clear holding by the defender is.