I wouldn't pay much attention to such statement knowing the historical background. Stallman was and is a proponent of his own terminology, i.e. "free software", and the related definition which actually isn't that different (hence the catch-all "FLOSS").
Anyway, it's stupid to argue over this. The FAQ on the Commons Clause site itself, reflecting the intention and thinking of the drafter and the initiatiors, specifically states the license is "source-available", not "open source". When talking about open source as a form of licensing as opposed to e.g. software development culture/methodology, the distinction here is quite widely accepted and non-contentious.
Anyway, it's stupid to argue over this. The FAQ on the Commons Clause site itself, reflecting the intention and thinking of the drafter and the initiatiors, specifically states the license is "source-available", not "open source". When talking about open source as a form of licensing as opposed to e.g. software development culture/methodology, the distinction here is quite widely accepted and non-contentious.