So this is a sales objection. With regards to that justification, I'd want to know a) who in the business is generating it and b) what they expect to feed your answer into.
Is this just somebody who wanted to sound smart in a meeting? Then they don't need your source code; they need ~5 nice PowerPoint slides and you're done. I'd be polite but firm in this case.
Is this e.g. a risk management officer? Then OK, that is a totally reasonable ask. It's going to cost you $20k and some legal wrangling regarding non-disclosure and non-competes, but we will give up to 2 team members of yours up to one business day of access to our code in a clean room on our hardware, under supervision by a technical leader, with appropriate rules of engagement regarding questions.
Is this a business unit head sniffing for new things to build? Then I will not automatically eject out of this conversation, but my price just had another zero added to it, at least.