Most families living in urbanised locations had access to "communal gardens". For the general public, it'd be a tiny patch of land where you may build a tiny shack to put your tools in.

Now if you're working for a prestigious institution in high-up position, that patch of land gets bigger, you may be able to pull together resources to build a summer house there and the whole communal garden might be a prime location surrounding a lake, bordering a forest and so on.

This is a bit unclear. If every family had a dacha, and a family consist on average of 3-4 persons, then 25-33% of the total population had a dacha, consistent with the last part of the sentence.

The Russians I asked cannot recall anyone who didn't have a dacha in Soviet times. Maybe the dacha was legally owned by an uncle or by grand parents. But everyone had access to one.