Confusing “Icing conditions” (... conducive to the formation of...) with Freezing conditions (OAT below zero)?
Boeing terminology, requires EAI On in Icing conditions. Due to the local pressure drop, and the risk of nacelle icing.
Active Frost requires a temperature below 0 (either OAT or local surface temperature).
If the aircraft has flown already, some (wing) surfaces may still be below tank temperature as a result of cold soak.
To the OP: without knowing the full details, sounds like the prudent course of action. Taking the generic Type II holdover time for Freezing Fog (which is not the case as you reported), you would be also be covered for Active Frost - probably the most accurate category, based on your post.