To answer your q (btw it's not a popup), I tried to explain in my comment you're replying to:
> sometimes I do the thing that works/is quickest to do, move on, and then revisit when I have time
Knowing me, it was probably something where, at the time of building (2015), I saw elsewhere - naively assumed it was a good idea/it works, copied it and moved on. I didn't give it as much thought as I could have. Now it's a case to prioritise that and improve it, along with some other much needed optimisations.
I wouldn't worry about it. It's something almost all blogs do, because it works! HN is the minority in... just about everything, but particularly how they feel about folks promoting their own work. 99.9% of readers just close the popup if they don't want to sign up. You're not driving anyone away who wasn't going to pick some other random thing to get overly angry about anyway.
So then the question arises: why do you have a popup on your site that does exactly that?