ETH didn't call him a liar, or call him lazy. The fact YOU have to keep lying or exaggerating about what he said to make it sound bad proves that there wasn't actually anything wrong with it.
All ETH said is that Sancho wasn't selected due to his performance in training as we have other choices. He even specifically said it was only for that game. ETH didn't give any details, so we don't know if it was a lack of effort, his regular late show, or whether it was more due to form as he was trying his heart out but just nothing was coming off for him.
If your boss at work says another employer performed better than you this week so therefore he gets selected to represent the company, you'd throw a tantrum and publicly call your boss a liar?
Those examples are very off. In truth we know almost nothing about the circumstances so everybody should be a bit more careful with their assessments unless they have background information.
Right now we know
- Sancho suffered from online (racial) abuse after the penalty shootout with England
- ETH sent him to a mysterious training camp in the Netherlands to work an his 'physical and mental issues'
- ETH left him out of the squad, answered it was because of his training performance and Sancho disagreed with that publicly, claiming to be scapegoated
- ETH values discipline very highly, Sancho seems to have had issues with his punctuality
The rest is up to interpretation and depending what happened behind the scenes, within the room of interpretation the facts leave us, there are scenarios imaginable in which both reactions are understandable.
If Sancho indeed suffered from a mental illness, Ten Hag's way of gandling this was disastrous. If the 'mental issues' he spoke about when Sancho was in the Netherlands were actually not health related but rather disciplinary it is a different topic. But then a measurement such as this seemed a bit out of proportion as well.
One way or another, there are hundreds of players like Sancho who aren't the most disciplined out there and IMO it should have you thinking that you rarely if ever see a manager clash with a player like this. One quality of a top manager is that he knows how to handle different characters and egos, that he knows how to get different personalities behind him and how he keeps players who are not in the starting line up or go through a somewhat frustrating episode motivated and loyal.