He slipped on the pitch alright but in doing so he carried on and fouled the attacker who was in a goalscoring position so its a red.
Changing rules to suit a slip will open a can of worms. Players claiming they slipped, faking stumbles, slipping and carrying on ito the attacker deliberately etc. Professional game is full of fake,injury playacting, trying to get an advantage etc. If it was other way about we would want the opposition player sent off for stopping us scoring.
Keep it simple no interpretation by ref or var what caused it a foul is a foul.