If it's a dll it could just as well be a skse plugin.
What's currently there is basically a heuristic but it has to be such that if there is any doubt it has to fall back to installing to data under the assumption that it was packaged for existing mod managers.
That's why the enb modtype is currently disabled, there are regular mods around that include a "suggested" enb in some subdirectory that confused the detection so the mod installed incorrectly.
In the future the "mod type" could simply be something set up on the website by the mod author and then vortex doesn't have to guess.
For the moment it would probably be better to look at the category as an additional hint (ok, it looks like an enb and it is in the enb category, then it might actually be an enb)