"best margins" suggests highest profit as percentages of retail price right?

That sounds wrong. Toilet paper isn't a traditionally high margin thing, and I doubt they'd be able to wring out much more margin by buying the factory, or else everyone would do it. High margin products traditionally are either the luxury products, low volume products, or convenient/convenience products (chocolates by the till, sandwiches to go).

Now its possible that toilet roll is their most profitable product in absolute terms. Everyone needs toilet roll so they probably sell a lot.

I still buy the Scott brand. It’s cheaper than Kirkland.

And it’s high quality TP, getting a lot of good reviews and lauded by Wirecutter etc online. I keep a roll on the spool for after my bidet does the real work.