Check that users comment history. It's not that the joke went over my head, I just don't like the Gschmäckle that accompanies it.

https://www.reddit.com/r/gaming/comments/11dqe45/comment/jaa5y25/?utm_source=share&utm_medium=web2x&context=3

That statement was in a seperate comment so I don't fault you for ignoring it, but "story completion percentage" can only be "achievement data" anyways due to factors stated here:

https://www.reddit.com/r/gaming/comments/11dqe45/comment/jackn4w/?utm_source=share&utm_medium=web2x&context=3

It states it uses achievement data and that is how achievement data works.

Let me walk you through the steps that are the default in the industry:

1.) A steam account (or PS or xbox account) purchases a copy of a game.

2.) If that account at any time fulfills the condition for an achievement (like finishing chapter 1), that account sets a flag that it has reached that milestone.

3.) you divide the amount of flags by the number of copies sold.

That is it. That is how you correlate copies sold to the chapters that are finished. While it does not you an idea how many people finished the chapters, you do know how many copies have reached a certain progress.

You could write a custom reporting tool that tells rockstar when a new campaign starts and how it is going, but there simply isn't a reason to assume rockstar did this, especially since that would require you to be always online.

That isn't how that works. Those statistics are per unit sold, not per started playthrough. If your account completed a step once, your copy counts towards "completed".

I think the best description of what I see in the screenshot is a cross-promotion.

Not everything that fits some description of "hidden" is an easter egg.