Reviewing the article, it describes Loblaw, Sobeys, and Metro as the "three largest" firms accounting for grocery conglomerates, which implies there are other firms in the grocery industry up there. Since there is more than one parent firm, this describes–at worst, an oligopoly. Oligopolies do exert control over prices by virtue of the few suppliers in the market, but their price-setting isn't monopolistic.

To the first point you've mentioned, my argument is towards support of price-discrimination, and not monopolies. The article does indeed demonstrate third-degree price discrimination (same product, different store/market segment, different price), but I did not try to connect these two.

To the second point, the reason for there to be an oligopolistic market is the natural result of an industry that has the kinds of barriers-to-entry that a grocery store seems like it might have: the substantial investment required to purchase the initial inventory, the real estate, and the labor costs.

With respect to Canadian consumer protections, I have no input.

That’s so kind, thank you! I am studying economics and the discussion helped reinforce a lot of fundamental learning.