Kellogg’s have always advertised that they don’t make cereals for anyone else.
Even if they did, they wouldn’t simply repackage the premium product. It would be a diluted one, because the own labels have wafer-thin profit margins.
The reason for this is that supermarkets are not relying on the own-label products to make profits. They are there to tempt you in to store, so that you buy the branded products, which have much higher margins.
The comment about Tesco not allowing the use of ‘Tesco’s’ is standard practice for brand management across any business, which will have strict guidelines around logos, fonts and “tone of voice”, which is the language used in all their communications.