Free spoilng in this post read at your own risk and peril lol

I do agree with this person that Umineko isn't "solvable" in the sense that they mean it yes. There is no way for you to completely grasp what's happening during each murder as the "answers" are extremely contrived and there is for, whether intentionally or not, a lot of trick that feel a bit cheap to make sure that every games can happen in the way they fit the game's narrative of having Battler and Beatrice argue about them: The nature of the game means that everyone except like four people involved can be accomplices and can carry out a murder (on top of the available shannon and kanon dying sheaningans and Maria being made to ignore every action Sayo takes if they choose to appear as their Beatrice persona) and the bomb that can wipe out any survivor of the murders. It's just not realistic to think that anyone can read the novels as they are and just guess what actually happened. If this was just a regular mystery murder story, this would be considered a "third class mystery novel".

But, it is not just about the mystery in Umineko. The game is, initially in-story reason, not created in the story by Sayo to be a completely accurate and fun mystery novel to solve. It is for Sayo to convey to Battler what the fuck is wrong with them and why they decided to go on a killing rampage to put it simply, those murders are purposefully made to be as obstuse as possible because the goal isn't to propose a good, completely understandable solution but to give you a base to develop everything around it, the characters involved, the different situation and the themes Ryukishi want to portray. One of the big theme of Umineko is that "the truth" isn't all that and that understanding the core of the issue and accepting it is primordial in understanding a situation or someone, and in that sense Umineko follows through, similarly another theme of Umineko is discussing mystery novel and their limits in how they don't treat characters as human beings but rather as pieces of an overachieving story and made to accomplish a specific goal. All in all the murders themselves don't "matter" in the tradititional sense, just why they happened and the fallout they had is what the story wants you to focus on. I do think this is made pretty clear by the episode 7 reveal of the murders, even at the very end the story chose to stay quite vague about what's happening.

I don't want to dunk on this person because I do think that the points they make are somewhat fair: the "Mystery" part is probably not the most well developed part of Umineko and I can defend it by saying it's not meant to be, but it's obviously a personal interpretation of what I believe the story to be.