He made almost no money with open source. He made tons of money by withholding features from the open source library and instead only offering them under a commercial license as an add-on. Since this is itself a violation of the LGPL, anyone using the Pro version isn't even using open source at all, as both the original and Pro enhancements are at that point licensed with a commercial license.
An accurate title would be: "How to make $100K by writing an open source library, waiting for a lot of people to build a product using it, then holding back essential features until they pay you money, which they will do because they are suffering lock-in and it's way cheaper to pay you than pay a developer for dozens of hours of work to adopt an open source solution."