I could see bringing in Bolton (whom I despise) as Designated Bad Cop for negotiation purposes, but he clearly wasn’t willing to play the blustering foil to Trump.
Trump brought in Bolton, not because he necessarily agreed with him, but because he valued his insights. My guess is Bolton expected Trump to agree with him.
A good manager should have a team with diverse opinions, all making their case, and then make prudent decisions based on that input. Bolton’s role was to make the case from a “Hawkish” viewpoint, Trump obviously didn’t see it that way, in the end.