Because if (or when) Italy is about to introduce capital controls to prevent its banking system from collapsing, investors will not make a decision based on the industrial outlook of the UK, but based on the stability of its legal and banking system, and its independance from Italy's financial woes.
But the stability of the UK's legal and banking system receives precisely zero benefit from Brexit (quite the opposite) and the UK gains further "independence from Italy's financial woes" from Brexit only inasmuch as it would lose some of the trade it might be expected to lose in the event of a Eurozone recession a little earlier.
(I could just about see how the UK would benefit from being less exposed to Italian financial woes if you anticipated an EU policy response of requiring Member States to contribute to bailouts and/or buy up Italian debt, but that's the bit you ruled out, and I'd see as being unlikely to be applied to non-Eurozone states)
No, they were trying to rope in even non eurozone member states.