I do get pissed at people who drive 30mph through heavy fog. That's probably what he was pissed at. It's more dangerous to drive under the speed limit through low visibility than it is to drive the speed limit because people who are driving the limit won't see you in time to avoid you if you are driving really slow.
My guess is he was pissed because you were driving so slow.
Most (quite possibly all) jurisdictions include language regarding weather and/or road conditions WRT speed limits. Effectively, the speed limit sign has a line at the bottom that says 'under normal/ideal driving conditions'.
Low visibility, poor traction, etc can mean that the posted limit is an unsafe speed; in most cases, prevailing traffic speed would be the best available measure of this. One car driving at 20mph under the limit is acting in an unsafe manner, if everyone else demonstrates by their driving that they consider conditions fine. One car pushing forward at the limit while everyone around them has slowed down is probably the 'unsafe one'. This is a pretty low bar, considering that most people speed most of the time, and do not adequately consider conditions. So if the prevailing traffic speed was 30mph, conditions were likely pretty bad.
While you normally can't be ticketed for 'speeding' if you are under the limit, there can be situations where a careless driving charge applies even though you are driving at the posted limit.
I don't use hazards in a moving vehicle unless I am in heavy fog, and proceeding only far enough to find a safer place to pull off the road. I will occasionally put them on if there is some traffic condition in front of me that I think the car behind me can't see (since I'm often driving a truck and trailer that blocks their line of sight.
However, hazard lights indicate a hazard - something that you are going to overtake more quickly than 'normal'. It's why truckers use them on grades that are too steep for them to attain normal speed (most often when their on-ramp is followed immediately by an uphill section). It's hard to suggest that flashers @ 1/2 the speed limit are more dangerous than flashers @ stopped, since a car travelling at the limit will have twice the time before they pass the slower vehicle; it's not like they're driving backwards at you.