To the extent I am observing the details of this story, it seems that Corbyn is saying that in 2017, the US government talked to May's UK government about purchasing American pharmaceutical products.
I don't see the problem with that. The UK buys these products from everywhere. Why should I care where they come from? I only care if they are effective and not disproportionately expensive.
If they purchase US drugs non-exclusively, that's not selling the NHS. It's supplying it. And as a bulk purchaser, getting a good price.