The blog post is confusing, but the image is very clear.

5.2.0 was released. Then 5.2.1, 5.2.2, 5.2.3, 5.2.4, 5.2.5, and 5.2.6 were released as stable updates. Pretty straightforward.

After 5.2.0 came out, normal development continued toward the upcoming 5.3.0 in Linus’s mainline tree. As bugfixes for real problems (crashes, data corruption, build breaks, security issues, etc.) were written and merged into mainline, a subset of those fixes was then backported to the 5.2.y stable branch and released as 5.2.1, 5.2.2, and so on.

In other words, there is a separate 5.2.y branch, but most of its changes are not developed there first. They are developed in mainline (the code that will eventually become 5.3.0 and beyond) and then cherry-picked back into 5.2.y as “stable” bugfixes. There is no “merge 5.2.x back into 5.3.0”; instead, stable only takes fixes that are already in mainline.

This means that any fix you see in a 5.2.y release should already be present in the mainline code that leads to 5.3.0 (or replaced by an equivalent fix there). So when you move from 5.2.6 to 5.3.0, you should not lose any of the bugfixes you were getting from the 5.2.y stable series.