I've seen that many times and it is BS. Everyone I know who only owns one gun doesn't know squat about guns. I recognize this may not be true for every person who owns only one gun, especially for someone on a budget.
You contradicted yourself here.
It's not BS. I had only one gun for many years, my 4" S&W M586. I dedicated myself to learning how to run it and master it. Instead of buying lots of guns, I bought ammo and trained with that one revolver. I do like shooting other weapons and calibers of course, but when it comes to serious business, I always go for that 586.