Can someone tell me if I can use my ZED ticket on UA? I'm planning to fly HKG-SGN tomorrow.

The reason is I went to buy a ZED ticket today and when the counter girl asked me which carrier I'm going to take I said UA and she said "Well, UA does not have agreement with MH so I can't put UA as carrier on the ticket! So instead, I'll put CX there". If thats the case does that mean my ticket will not be valid on the UA flight? I checked the zedmiba website and UA is listed as a ZED member. Can someone explain this to me as I've already booked hotels etc

Thanks