The broader context is that approximately any country that wants a healthy, stable dairy industry (often regarded as a component of national security) has to provide government support to their farmers one way or another. “Stable” is particularly important for dairy because recovery when lots of dairy cattle are culled is expensive and takes years.

The US and Canada use different, incompatible means for accomplishing this, so you can arbitrarily paint either as “cheating” the other by saying “well (Canada/The US) does X to the tune of (a dollar amount) and the other does zero of that, they’re cheating!”, i.e. by lying by omission.

Because of the way the US subsidizes theirs, Canada could be vulnerable to dumping. So there are protections in place. This is not remotely a big deal so long as both sides are competent and operating on something resembling good faith…