Indeed, and I know quite a few myself (some ~25k) but not many log 18k hrs in GA.
Apart from some commercial opportunities most piston GA flight is not remunerated and to log 18k hrs you will spend of the order of US$2M (two million bucks) of your own money, after tax.
A typical airline pilot who flies piston GA also will not be flying IFR GA. Most of them get plenty of IFR at work and have zero interest in hacking IFR in piston singles or twins (I know this because I have flown with a fair number of them). They prefer to fly for fun, so they fly rag and tube types, homebuilts, etc. And they don't log 18k hrs in these.
So, not impossible - just exceedingly unusual.