Actually - I doubt it is possible to trademark the term "A2". Too short and too generic. And anyway - that's not what ATM did.
Here is ATM's trademark:
https://trademarks.justia.com/854/53/a2-85453431.html
To violate the trademark competitors would need to use an "a2" using specific defined colours and a particular shape:
There is no law stopping cows to produce A2 milk ... and I don't see any reasons why sellers would not be allowed to state this fact (if it is true). The A2 milk company has no monopoly on selling and marketing milk which contains only the A2 and not the A1 protein.
Competition is good ...