From what I understand, he didn't train today due to family reasons. He definitely was training for the Supercoppa games, he is even in the videos that Inter posted of their training sessions.
From what I understand, he had different training compared to the rest of the team though due to his physical issues - I saw a fan post this who was able to see Inter's training sessions in Saudi Arabia that Dumfries was going through different exercises.
I feel a lot more is being made out of this in assuming Dumfries is refusing to train or not giving his all. The rumours about not reaching agreement for contract came early in December when he was injured. Once he came back from injury, he started the first game in January versus Verona. During that game, he needed physical attention in the first half (similar to Lautaro vs. Bologna actually when he re-aggravated his injury and missed the next two games). I think it's simply a case of him having physical issues. Even for the Monza game, he was actually predicted to start by SkySport who have been pretty reliable with lineups. It was only changed a day before when (I believe) Guarro mentioned Darmian will start as Dumfries still has physical issues. Even in November, he had to skip a few games here and there (I remember the away game vs. Salzburg) as Inzaghi mentioned he was struggling.
I feel if there's anything we can't question about Dumfries it's his workrate and committment. He has other weaknesses but the guy has a great mentality and works very hard.