Often tho I realise that my mates, who do like good music and basically just listen to 90s deep house and garage as is sensible, will mention something about, I don't know, Television or Dylan or something and I'll realise that somehow, sometime, they must have squirreled themselves away and worked through the canon at one point.
Hours and hours of work must have been going on behind closed doors. Weekends spent diligently listening to music you don't really enjoy just in order to have built up that depth of knowledge.
It's the music equivalent of being "well read" isn't it. They know the classics.