You're describing the primary selection. The article's about patching secondary selection back into gtk3 (c. 2015?). It's ctrl+select followed by ctrl+middle to paste as yet another clipboard. This has the unique and useful property that neither selection nor paste changes the text cursor's position.

A more in-depth look at secondary selection is in the embedded video which has it's own write-up on https://www.cs.man.ac.uk/~lindsec/secondary.html

[I suspect primary-secondardy selection is not a confusion you have, but one a casual skip-the-article-direct-to-comments reader might have]