Government's mistake was to say issues aren't "up for discussion" instead of "we won't make concessions."
You put yourself in the position of either not even letting the other side explain their interests or of contradicting your promises, right from the start.
That's a valid opinion, but let's be honest and admit there's literally nothing Boris Johnson, DIT or anyone else could do at this point to make you stop holding it.
Whether the issues arose in some meetings DIT staff took with their counterparts (or even industry) before negotiations even started doesn't change that calculus one iota.