I'm inclined to agree, and I dropped the expectation that the average level in a D&D game world was 1st level back in the early 90's. Exposure to the FR didn't make me admire the FR as a setting, but it did force me to question the sacredness of my cows, and Gygaxian demographics was one of the things that went away, not the least of which is that Gygax himself didn't seem to really follow his own advice.In my travels, which includes living on three continents, I've toiled next to all manner of folk, and your blanket textbook assessment above is, in my humble opinion, inaccurate. Many 'skilled' people that I have had the honor of working beside had no degrees, little in the way of education; they learned their trades by application, not in a classroom. And this has been the way of the worker since time immemorial.
In my opinion, the average modern person is best modelled in D&D terms as a 2nd or 3rd level Expert. Granted, in the real world we don't have hit points, but a reasonable approximation - casual realism if you will - can still be made by assuming that average CON in the modern world is below 10 owing to our relatively sedentary lives and the fact that low CON individuals aren't winnowed out of the population with quite the frequency that they were before modern medicine.
Similarly, I tend to use demographics that models the average mature adult as a 2nd or 3rd level NPC classed individual (usually commoners or experts) which lower ability scores, no focus on combat skills, and fewer advantages than a PC. Thus, even though the PC's are lower level starting out than most of the population, they are with respect to combat skills typically quite remarkable even so - sufficiently so that they can reasonably take on challenges that most NPCs could not. On the other hand, they tend to be less competent starting out compared to NPCs in handling ordinary day to day challenges of commerce and farming, so it's pretty natural that they are looking to something other than trade as a means of survival. This also has the nice side effect that NPC's are easily exploitable nor are they completely helpless, so the PC's have motivation to perceive "the dungeon" rather than the town as were the action and profit is, and from a simulationist perspective the loose balance between the dungeon and the town is explained - neither side can easily destroy the other without heavy losses. It's the PC's and their PC's foils - the BBEG and his minions - that upset this balance and so drive the conflict.
As for your personal experience, I don't think you are necessarily 'multi-classed'. I just think you have several levels in some sort of NPC skill-money class like Expert, giving you ranks in a relatively large number of trade skills. Now, the guy I knew who got a PhD and then joined the Rangers to become a Combat Entomologist - he was multi-classed.