It's not price fixing. It's what's stocked by big retailers.
Real world example.
Heinz Baked Beans, UK. There are tons of other baked beans brands, but almost every store has Heinz. It dominates the shelf space by a huge margin.
Heinz is specifically called out in the article. They've made massive hikes in price, everyone's complaining about it, but often it's the only product on the shelf for a common, quick meal ingredient in UK meals.
There is no choice. It's not a simple economic choice. Most people can't choose the cheaper brand as often there's no choice.
Heinz Tomato Ketchup is/was in an even more dominant position. Many mid-sized grocery stores literally only stocked Heinz. (the co-op one is a pretty good substitute btw).
While theoretically there is consumer choice in the market, the reality is that it's wasn't true unless you want to go round a bunch of different shops.
In the long run Heinz has done some serious damage to their brand, but short/mid term are making juicy profits as the market is so slow to correct due to the way supermarkets work. How shelf space is allocated. By how the industry has become a small amount of ridiculously large companies hiding behind a patchwork of acquired brands.