SNA is slot controlled.
However, it doesn't matter because this "new" PHL-SNA isn't really new. If you'll notice PIT-SNA, which is currently 2x daily, goes to 1x daily in June. All US is doing is shifting the flight from PIT to PHL. Given the stronger O+D of PHL, this move might make sense. However, the move also serves to weaken the connecting power to PIT.