I assume it's because he thinks bitcoins are about as "real" as pixels on a computer. Perhaps he thinks "real" money is better because he can hold it, and imagines all money is actually printed on bits of paper and metal coins. If that were the case then he's making some pretty foolish assumptions in his attempt at humour.
I guess the joke's on him.
I never did like Steve Martin's humour.
Steve Martin is a well known prick. If he didn't work with comedic legends no one would even know he existed.