Certainly you're right that the supermarket chains have the opportunity to freeze prices on anything they sell. That's obviously within their purview. In this case Loblaws chose to freeze the prices for their own in-house brands, which of course are not made by Loblaws. They are made by other manufacturers and processors, and in some cases the same ones that produce the brand-name products.
I don't have any information from my vantage point on the returns that those processors get from the supermarkets on their brand name versus no-name products, and what the difference in cost of production and quality for the no-name versions of their output would be. But I think it is recognized that no-name brands do tend to have higher profit margins for the supermarkets than the overall portfolio of products.
There were also lots of questions asked, fairly in my judgment, about the significance or meaningfulness of that no-name price freeze anyway, which of course now has been done.