I agree it's weird, but from what I understand the devs want to make HOI4 into this grand (no pun intended) "flagship title", so I guess they want it to be as polished as humanly possible? Also the devs have said they tried a lot of ambitious new things, like the battle planner, some of which turned out to be way harder than they first thought they would be. They've also admitted they did announce it way too early.
81 days seems a
loooong wait for me, too, and I wish it was finished sooner, but them's the grapes, I guess.
I'm cautiously hyped for Stellaris, so I don't know if I mind getting that first. I'm more worried about how I'm going to afford buying them both in such a short time-span

. Maybe PDX could release a cheaper "Home Edition" of both games

.
It means hoi4 is arbitrarily delayed so sales don't coincide.
No, hoi4 would be out a month or a little less before June 6th if there was no stellaris. Not huge but Stellaris most certainly affected Hoi4's release date.
Based on what? Don't throw out rooster-sure statements like this without anything to base them on. That's how those pesky myths and misconceptions originate.
I think that if one of them was to be delayed, it'd be Stellaris since it was announced last of the two.