Is tinned tuna ultra-processed?

Not ultra-processed, and which group it lands in is decided entirely by what is in the tin with it.

• Tuna in spring water: group 1. Fish and water, and cooking in the tin adds nothing.
• Tuna in brine: group 3, since salt is a group 2 ingredient.
• Tuna in sunflower or olive oil: group 3, for the same reason.
• Tuna with mayonnaise or a sauce: group 4, from the stabilisers and flavourings in the dressing.
• Flavoured tins, lemon and pepper or similar: group 4.

Retorting is not a marker

Canning cooks the contents inside a sealed tin at high temperature, which is what makes it shelf stable for years. It is a heat treatment and adds nothing, so it does not move a food any more than pasteurising milk does.

Tinned pulses, tinned tomatoes and tinned fish are all in the same position, and the tin is doing the work that additives would otherwise have to.

Working this out from a receipt is harder than from a packet, because a till prints about twenty characters of product name and no ingredients list. That is the problem Bagged exists to solve, and how a receipt is structured explains why it is the hard part.