'source available' means the source code is available. Open source comes with a whole set of guarantees [1] about free redistribution and derived works.

Copyleft licenses like the GPL come with extra guarantees that do not violate the core guarantees of open source software. Instead, they make them stronger. The 'restrictions' GPL imposes essentially boil down to this: "if you use (parts of) GPL software, you must give your users the same freedoms the GPL guarantees." GPLv3 and AGPL closed up loopholes that allowed people to bypass those clauses.

[1] https://opensource.org/osd

Your link is to a politicized advocacy group, the OSI. Are you above, say, forty years old? You should remember, then, the arguments over introducing "libre" as a term because "open source" didn't describe those free redistribution rights.