i agree with you that on a technical side of things, it is possible to separate DLC engine from base game engine.
It's not a matter of having a "DLC engine" and a "base game engine". It's a matter of branching the whole game client codebase with each major evolution and/or engine change, and having essentially multiple codebases that are very similar but separate and unique, and which
will diverge over time except for parts which are carefully backported.
I have worked on codebases like that, and the fact that game engines are involved is completely secondary, the same applies to boring old business applications, libraries and frameworks. Staying within the gamedev context, it's easy to imagine having a branch that's still built using Unity 5 while your "latest & greatest" is on Unity 2019.3. Sure, you can port some feature code to the legacy branch, but each build is a completely separate application. You use a launcher app to make the user pick a "DLC" which essentially selects one of two standalone client builds.
So; the legacy branches get some scraps and some mandatory back end compatibility updates from the master branch, but all the good stuff is built on tech that simply isn't available in the legacy branches.
trust me, i know a thing or two about this stuff since im a game dev myself.
I built real-time 3D rendering engines used in commercial applications back when everybody used to roll their own (early 00's), and I've been working full time with UE4 and Unity since 2015. Two patents on 3D rendering tech and one on multiplayer networking. I also know a thing or two about this stuff.
But wild epeen swinging aside, I still claim the following: Odyssey could be Pac-Man from base game/Horizon perspective, we cannot assume anything as to what can be reasonably ported across. Only FDev knows for sure.