Having interviewed about a hundred programmers who responded to a job search specifying that we're looking for "brilliant coders", I can objectively say that 90% are below average...
Seriously though, your perception depends very much on the quality of your peers. I'm lucky enough to be immersed among the very best, so the average developers I regularly interview seem - to me - incapable of programming their way out of a paper bag, even though they hold normal positions in normal companies, presumably successfully.
90% can be below average. It depends on how you define 'average.' If you're using the median, then 'below average' can only be 50%. But if you use the mean (or its variants), then you can have 90% of a population be 'below average' if the population is very skewed.
Also, he was talking about 90% of coders he interviews. Of course they'd be below average - if they were above average, someone would hire them and they'd leave the job poll quickly. Such is the pitfalls of sampling a non-random population...
Seriously though, your perception depends very much on the quality of your peers. I'm lucky enough to be immersed among the very best, so the average developers I regularly interview seem - to me - incapable of programming their way out of a paper bag, even though they hold normal positions in normal companies, presumably successfully.