So this is going to depend upon the transformer design. Again, a Class A design is gapped and has a larger core to avoid saturation. Are the UTC output transformers able to handle the full Class AB power rating when operating in Class A? As you noted, no. Because the transformer design is a hybrid between Class A and Class AB. Works fine for Class AB, at a higher price point, but Class A can't run at the same power.
This is conflating single ended with class A as none of this is true of PP class A.