This is not true on just so many levels. Lots of people have mothers not worthy of the name. I don't know exactly what fatherhood being a theory and motherhood being a fact means but I struggle to find any that isn't offensive to fathers.
O'Rourke was a conservative satirist (back when that could even be a thing), was semi-famous for his long term appearances on NPR's Wait Wait Don't Tell Me, and was an advocate for gonzo journalism.
I take his quote to mean that motherhood is more difficult than fatherhood, especially back in the time he was active. The quote should, I feel, not be observed as a factual statement, but rather in the vein of his politically based humor. Largely, it was a simpler time.
I was a stay at home dad for a number of years and my observation is that kids never get over a bad relationship with their mother (also informed by knowing a number of adults with mothers that weren’t that good for them); any one can benefit from a good relationship with their father, but it’s a little more optional for the most kids. Ready to benefit from a supportive dad but ready to go it without that sort of helpfulness.