Knife-Edge - the decision on whether to de-ice is a complicated one. If your a/c had a longish inbound sector the wings would have been well below zero and any rain would freeze on them (known as 'rain ice'). Often an upload of 'warm' fuel (normally well above ambient temperature since it is normally stored underground) will clear the ice. The Ryanair may have had a short inbound sector or uplifted fuel - or both. One hopes that consideration was given to de-icing and I'm sure it was. Normally a combination of experience, indicated wing fuel temperature and a look at the way the rain behaves as it lies on the wing will make the decision. No doubt your Tom Captain made the correct decision
