Alexei Sayle: 'Austerity is the idea that the global financial crash of 2008 was caused by there being too many libraries in Wolverhampton.'
Many a true word spoken in jest, etc ...
I interpret that phrase to mean "the speaker thinks they're joking but inadvertently says the truth" but Alexei Sayle tells the truth and knows it, but he makes the truth sound like a joke so people will see how ludicrous the truth is.