You were given good advice by that farmer, but young bulls are rare, normally you will meet Bullocks, these have been castrated. If we learn to use the correct terminology it will perhaps help to lessen the myths that build up around the real instances of aggressive animals.
The unnecessary warning of a permanent sign that bears no relevance to present risk, should be commented on. At the least, it is a sign of lazy land management and too often a sign of latent hostility.
I once found a Beware Bull sign on the gate of a barley stubble field with a volunteer potato plant on the field headland. At a minimum rotation, after grass, would suggest 5 if not 6 years since there had been a bull in that field.
The OP is giving a useful insight into the mixed messages visitors are getting from the occupiers of our countryside. Not enough educational input is going into the agricultural colleges about access; Farmers cannot continue to rely on the excuse that they they work harder and longer hours, therefore these small items of bad management are excusable.
These signs of bad manners and latent hostility are a result of the Farmer deferring to the Landowner interest in matters of land management, because of the conflict over subscriptions. More research should be going into public access and the results used to train young farmers and stockmen to respect their customers, when they visit their places of food production.