And how long did it take you? I don't get this pointless bullshit argument. If you haven't killed it yourself, how can you refute what top guilds are saying? When the few guilds that have killed it say "This fight was undertuned" on top of Blizzard essentially stating that, I'm going to believe them.
What the actual fuck is this reasoning? Difficulty is objective. It quite literally is. Fights require certain DPS/HPS requirements on top of proper handling of mechanics. Just because there's gaps in skill between players doesn't mean that Mythic isn't OBJECTIVELY harder than heroic. Likewise, certain fights are OBJECTIVELY more difficult than others. M-Ursoc is a joke if your raid can meet the DPS requirements. It's essentially got 2 mechanics and very slight movement. It's close to patchwerk. Meanwhile, M-Il'gynoth is something completely different. M-Il'gynoth is objectively harder than M-Ursoc and, according to basically everyone that has done M-Xavius as well as Blizzard themselves, M-Xavius is too easy.
Oh and just so we're clear on the whole "difficulty is not objective" bullshit. A guild that struggles with normal would find heroic and mythic objectively harder than normal.