Hm. I did not know he held that view.

It does make sense. It sounds like he's against things like Magento/Gitlab/Alfresco which have totally open source versions and commercial/enterprise versions with closed extensions. (I realize those aren't the best examples as they're not GPL).

With the dual licensed product, you simply have two licenses for the exact same code base. If you want to reuse it, you can, but you need to release your derivative works back to the community. But the people/organisation behind the tools still have to live in this capitalist world that's far from the free software paradise of the FSF. Developing under a dual license can provide a revenue stream back to the writers to help keep that work going.

Just as a remark: He doesn't see Free Software as anti-capitalistic. He i.e. also supports the possibility of selling GPL software. (just as in the 90ies, where downloading large files from the internet was hard and it was common to buy a box with S.u.S.E. Linux)

"Many people believe that the spirit of the GNU Project is that you should not charge money for distributing copies of software, or that you should charge as little as possible—just enough to cover the cost. This is a misunderstanding. Actually, we encourage people who redistribute free software to charge as much as they wish or can. If a license does not permit users to make copies and sell them, it is a nonfree license." https://www.gnu.org/philosophy/selling.html

What he cares about is that the software is free as in freedom and contains no lock-in: You can look at it, you can edit it, you can distribute it.