Presumably that is on the assumption that Scotland would not be a member of NATO, which obviously does include Italy.
Unlikely to be a NATO issue. More likely, as with shipbuilding, to be down to the fact that under EU competition regulations, nations such as the UK can give preference to in-country defence contractors but can't give preference to any foreign contractor.
In other words, a Ferranti or similar defence company based in an independent Scotland would have to compete on cost with other bidders across the EU, and could find competitors in the remaining-UK being given preference over them for remaining-UK defence contracts.