But it does not hold true if the pilot does not escape icing conditions if they are found.
So, yes, an airplane not certificated for flight in icing conditions is essentially prevented from flight in IMC for that portion of the year when air temperatures are likely to be near or below freezing at the altitudes the pilot wishes to fly.
To the extent there's an FAA definition of known icing, it's in this paragraph from a 2009 letter from the agency to AOPA: