> It does not mean it is free and open-source (FOSS).
When the campaign to describe software as "open source" kicked off in 1998, it consciously choose the Debian Free Software Guidelines (DFSG) as the basis for the Open Source Definition (OSD), so that "open source" would refer to the same thing as "free software".
Clearly the connotations of the two terms have diverged quite a lot, but their denotations weren't meant to.
Companies that want to publish their source code but not give you the right to use it however you want, e.g. Cockroach or (my former employer) Materialize, are described as “source-available” instead.
The problem is that "Free" means two things in English, which is why some like to use the French/Spanish "Libre" instead, to separate the "free-as-in-speech" from "free-as-in-beer".
This includes the point the person you are responding to is trying to make, in which case they are noting that open source and source available are not the same things. Their point seems to be that open source, by the very definition to most who matter/care about the concept, implies the "free/libre/disentangled" portion. Whether that means derivative continuation (GPL and its kin) or not (MIT, BSD, etc).
This is why the phrase "source available" exists.