Yep, most Tesco home delivery stuff is literally a Tesco assistant walking around the normal shop, picking up your stuff and putting it in the bag. There's a limit to how many substitutions a shop will make before the order becomes impossible - particularly when basics like milk, eggs, loo roll, soap shelves are empty.
It's why Morrisons (and presumably other supermarkets) are trying to set up packing lines to put together standard hampers of stuff for delivery. That's such a change in their systems though that it'll take a week or two before we see it.
Meanwhile all the best to your in-laws I hope they get better news (and what they need) soon.