To the extent that he missed the mark, it's not so much that the Tigers lack talent generally (they are clearly a bottom third farm system right now) more than there are a handful of systems out there who appear on the surface to actually have less at the moment.
The Braves, to their credit, have graduated a ton of talent into the big leagues over the past couple of years and have seen a lot of success with them (including a World Series, which is the point of all of this lol), but objectively, at this current moment, is the talent in the system actually better than the Tigers? Peoples' mileage will vary, but at least one of Keith's counterparts (Kiley McDaniel) doesn't think so, as he ranked their system dead last.
In an article about the Braves' ranking, Keith did also state that "I don’t think anyone’s rankings matter – where I placed Atlanta’s system has no bearing at all on the players or the team. It’s just my highly subjective opinion." I assume the same extends to the Tigers - people place a lot of weight on Law's opinions, but other people in the profession may see things differently as well and should be taken into account.