I've struggled with this popup (keeping it or nixing it; I'm the owner of the site). Sadly, it's insanely effective at converting folks (thousand of subscriptions over the last few months; high retention as I track those subs separately). It shouldn't trigger while reading unless you leave the window. And also won't trigger until you're at the bottom of the screen. And if you dismiss it once, you should never see it again with that browser on the site.

That said, it is annoying, especially on a page about newsletters. So I just modified my templates to take a variable to turn it off on a page-by-page basis (using Hugo to generate the site). Won't appear for this post anymore.