My understanding of the latest-gen Pdox games is that parts of the AI are shared at the level of the Clausewitz engine, and that Clausewitz in turn was developed for EU3, but designed as an extremely general engine for 2D games, not even just GSGs. So in some ways the HOI4 AI, for instance, is using techniques that might have worked great for EU3 (and, in my experience, worked fairly well in Victoria II, if you treat the economy separately), but are now unsuitable.

I'm interested as to whether they've improved the AI in Imperator - not that I'm going to buy it - as the Jomini sub-engine was supposed to streamline a lot of the "cruft" that had built up between 2007 and now.

As to your point RE: OPMs going on world conquests, I think that's part of Paradox's cross-franchise decision to move away from historicality and towards more "casual" gameplay (i.e., map painting), not just between EU3 - 4 but HOI3 - 4 as well; and ultimately has more to do with their balance choices for the player than the strength of the AI.

I do think that, as it stands, HOI4 and EU4 are at the same level as certain competitive games, where instead of trying to play historically, many players just wait for the next patch's balancing to over-correct in favour of some set of poor AI choices, and possibly for Reman's Paradox to explain it to them, and then exploit that to their advantage. Making the AI more competent, and removing the need for balancing and overcorrecting, would remove that perceived ability to exploit a broken enemy, and thus some of the desire from the playerbase for map-painting over historical accuracy.