'Isn't it tantamount to price fixing?'
How do you make that out?
In any market, if there is little or no product differentiation, then prices will tend towards the same figure. That's true of electric cable (my background) just as much as it is of ordinary litres of milk or loaves of bread.
It is also true of Heinz Tomato Soup but not of tomato soup. Tesco couldn't sell HTS at £1 if Sainsburys had it at 50p. But it is harder to work out the equivalence of own brands, premium or otherwise.
I think Sainsburys were hoping for an 'official' ruling from the ASA that its product was better.