So what does this mean?
"Flights marketed by non-Star Alliance carriers and operated by Air China are not valid for Mileage Plus accrual."
It means, if a flight is OPERATED by Air China, and if the flight number you booked is not of an airline in Star Alliance, then no miles are accrued.
For example, CX puts their code on a CA flight between HKG-PEK.
CX 6118 is the marketed flight number, and CA 118 is the operating flight number.
If you happened to book CX 6118, you get no UA miles. If you book CA 118, you get miles.
At least that's what the text you quoted says.