It’s a jocose remark.
It’s a corollary of an age-old adage that goes: if you’re a jack of all trades, then you’re a master of none.
If you want to be an expert, you need to narrow your field of expertise and learn a lot about it. The more you narrow it, the more you know about it (because you have less and less to learn about), until you narrow it down to nothing and know absolutely everything about nothing.
Well - er- yes but I thought you were poking fun at the title ‘expert’ that people give themselves. Whereas the whole world acknowledges that David Crystal is an expert in his field, I can’t imagine him describing himself as such.