The problem with Corbyn is that he has spent his entire political career opposing everything and as a consequence has never been involved in trade negotiations.
The US are allowed and are entitled to state what they would like to negotiate on. The UK are then entitled to do the same. After much backwards and forwards and time, an agreement is eventually hammered out.
The access to the UK market for their pharmaceutical companies is and would always be part of a trade negotiation and will be part of all negotiations with the US, Europe and all the other countries we will have to negotiate because of Brexit.
If he does not understand this, then it is just another reason for his not being fit for office.