its very vague to call them as designers
'designer' is a very vague term to begin with.
I, as a hobbyist, prefer the term "game mechanic engineer" for gameplay-related mechanics. Note: I don't consider myself a game-mechanic engineer, nor do I want to be one. (I consider myself more of a hobbyist world-designer, and I'm also a programmer)
because they only cover topics that they feel are important to the industry
They put out a video or two a week - so, they can't possibly cover everything. Are they supposed to cover everything, but poorly, or cover a sub-set of things, but more skillfully?
which is arguablly again vague seeing that most of their discussions aren't fully thought out.
They don't have discussions, they make videos giving their views and opinions and experiences. A discussion is what you and I are having right now - a give and take of opinions. Their videos are pre-written presentations of their views.
And many times, they cover the major points, but don't feel the need to cover every tiny ramification - which I'm glad for. I don't need them to make videos that are 15-20 minutes (instead of 5 minutes) to cover more obvious results.
Like Witcher 3 is the best detective game? Really?
I haven't watched that specific video (or played that specific game), but usually they don't make as absolute statements as the title of that video. But again, they are presenting their opinions, explaining why they think that way, revealing different lines of thought from my own perspectives, which are very valuable from a design standpoint.
Occasionally, they deliberately make certain statements to provoke thought, and then follow up in a later episode (they mention this in the "God does not play dice" episode). But sometimes their views are just different than mine.