That still does not sound like a conflict of any kind, practical or mechanical; If they wanted to use tech-prereqs, then the finisher would not be available until after the appropriate tech. Simple.
Except that, as I explained, tech prerequisites by their very nature make things more linear. If you give all of the policies within a branch different tech prereqs, then you can pretty much assume that they'll be selected in order of techs in most cases, at least for the branches the player goes into as soon as possible. This directly opposes the idea of a Finisher, which is that no matter how you end the tree you get the extra bonus. Finishers are built around the idea that there are multiple ways you could end the branch.
Think about it this way. Let's say there are three policies in a given tree that are "end" policies, where they all depend on the first few. Their effects are A, B, and C. Then in this patch we add a finisher with effect D.
Sometimes, you'll take A, then B, then C, or B-A-C. In those cases, selecting C actually gives you C+D.
Sometimes, you'll take A-C-B, or C-A-B. In those cases, selecting B actually gives you B+D.
And sometimes, you take A last and get effectively A+D.
The point is that you have multiple ways to get D, so it needs to be kept separate from A, B, and C. This is where Finishers work well.
But now, create an environment where C is always the last to be picked. Before, this wouldn't be a big deal, but now taking that final policy always gives C+D. At that point, there's no benefit to having the Finisher at all; you might as well just give that one policy both effects and be done with it.
Now, what can cause C to always be picked after A and B? Well, you could rearrange the dependencies within a branch, of course, but tech prerequisites do almost the same thing. If most of the Tradition policies unlock in the Ancient Era but the last one unlocks in, say, the Industrial, then even if it's remotely possible for that one to not be last, in practice it'll always be the one that triggers the Finisher. My mod is just an extreme example of this case, but it'll come up in any mod that uses the already-existing tech prerequisite mechanism for policies, hence my objection.
If we weren't supposed to be able to give tech prereqs to Policies, then we should never have had that stub, and if we ARE supposed to be able to do so, then the onus is on the developers to not add an additional system that directly contradicts the use of that stub. It's not about any one mod, it's about a flaw in the underlying design.