Yes, the US is no stranger to government influence on the prices of ag products, but my impression is that Canada is a couple notches above, especially the highly populous Ontario. Any comparison of the two needs to take that into account.

Based on a brief look by my layman eyes, it does not seem like Canadian grocery chains are vastly more concentrated relative to the US. It's hard to compare because so many of the chains are owned by the same few brands, but this chart is helpful:

https://www.howtocook.recipes/the-largest-grocery-stores-and...

I'd look at the top ten and then ignore Walgreens and CVS. I think you've got 8 conglomerates serving the vast majority of the 330M Americans. (Unclear to me whether Target should be included; I don't know how much of the sales listed there are actual groceries.) The OP article suggest 5 conglomerates serve the large majority of the 38M Canadians.