You don't know that. Things are changing very fast in Saudi Arabia.
King Salman has transferred every title he had to Mohammed bin Salman except one, the title "Guardian of the two holy mosques." That he is keeping. I think that is very important, and is going over the heads of most outsiders.
Saudi Arabia is changing along the lines of the old Papal states, when the Pope gave up most of the land it owned, which then formed the modern state of Italy, and kept just enough land for the Vatican.
Saudi Arabia has given up guardianship of the two holy mosques, and is becoming multi religious. Mohammed bin Salman could not have bought that painting if he was the guardian, but since he isn't, he can. This purchase was a statement.
islam cannot gracefully allow that to happen.