Everything needs to be considered in the round, as they say.
There are several mutually incompatible drivers at work here, and it may be wise to quietly let them do the heavy lifting and cancel each other out without making a fuss.
If Trumps immigration rhetoric is even half implemented, then US farming is fucked. Prices will shoot up and any notion of undercutting other sources is also fucked. Whilst there is a danger the UK could import cheap shit food from the US, we are much less likely to import expensive shit food. Especially given our own COL crisis.
If signing a token deal with the Mango Mussolini is enough to keep him quiet then it would be a strategic decision.
There is a lot of clever money looking to the midterms to jam Trump up - something his own side will be well aware of. So they are in a rush. And we know how easy it is to overlook the finer details in a rush.
Sadly UK farmers will probably be further shafted. But then at least they will be happy they it's what they wanted. Because if we were still in the EU ....