There are some decisions that if they referee doesn't award a penalty or a red card then he just being lenient with the player. If the referee decides to award the situation then he's also right.
In Saliba's situation, it's a red card. If the referee doesn't give it then it's his decision, but that doesn't mean he made a bad decision. Some referees are harsher than others.
Saliba was the last man, it's a known rule, if you're the last man and you stop somebody that is through on goal then it's a red card.
In that situation, I feel more angry at Saliba because he didn't need to make that challenge. There is a lot of space between the player and the box 18 meaning he has a lot of time to run up to the Bournemouth player and challenge for the ball legally, but he went for the foul. The referee was harsh, but it's still a red card offence.