Not true at all. In 2017, only 7.7% of contributions were unpaid [1], and it's been dropping for many years: "from 14.6 percent of contributions in 2012 to just 11.8 percent" in 2015 [2]
Maybe I should have been clearer. The payments to the contributors and the targets of their respective companies are not always identical. A team from redhat contributing to the kernel might not be remote and will have their own performance measurement stratergies.