The assassins and the dealer were in the suite one floor below bin-Salman's penthouse near the top floor. If they wanted to escape on a moment's notice they couldn't bring 10+ weapons with them. They went upstairs to shoot bin-Salman in his penthouse but wasn't there, just his security. They had a shootout, went downstairs, shot the dealer and then shot up the crowd to frame it as a mass shooting and cover up the assassination attempt.