From rumors I have heard GS develops releases side by side. So conceivably development on 17, alongside 16, is already underway. That means 15 could have started development after 13 was released. Just a conspiracy theory, no facts to back this up. But it would explain your timeline.
Another theory. Maybe this architect developed a prototype of the Shell tool that Graphisoft saw promise in and they developed further for wider release.
Or perhaps the original designs were done with a previous AC version with a bunch of existing methods but the project made a perfect poster shot of what the Shell can now do easily.
When it comes to matters of the heart and matters o marketing, there are no rules.