While the statement looks to hold truth, here's the actual stats this month:
Mental Herb 9.424%
Magic Coat 25.047%
Assuming they're not the same set that has both of these, that's a "whopping" 34.471% that Sableye can't completely stop. Meaning it cold-blocks 2/3 of all Deo-D sets in the weighted January stats. So not "tons".
actually, here's the real 1337 stats from january (post 92), which are a far better indication of the metagame than base level ou showdown stats. anyways, these stats clearly indicate mental herb was used 13.862% of the time and magic coat was used 27.578% of the time, adding up to about 41% of deoxys-d that now use your sableye as setup fodder for their hazards (and i'd be willing to bet that tournament usage of magic coat and mental herb is even higher than the 1337 stats show). not exactly a solid check. that's like saying bandtar is a solid check for gengar as long as it doesn't get hit by the focus blast first.
Still doesn't negate that fact that Starmie, number 12 on the ladder, kinda ruins most of the Deo-D+Offense teams (which, if you missed it, Deo-D is 39th on the January ladder...), assuming it's not the Choice Scarf Gengar-wielding 11.355% of teams Starmie has issues with.
thunderbolt is actually a thing, with an electric gem it's a straight ohko on most builds of starmie, and while not exactly common, the point remains that deo-d can run basically whatever it wants to counter specific stuff. also, deo-d is extremely commonly seen paired with bandtar, so the standard sequence is: deo-d set up sr as starmie comes in, deo-d switch to bandtar on the spin, bandtar uses pursuit and kills starmie regardless of what it does. again, not exactly a fantastic counter.
The fact that it can tailor its set to stop its would-be counters is also not a new thing to the world of OU. Say hello to Terrakion, who only can't touch Golurk with STAB and variable coverage moves in X-Scissor or Earthquake. Or Latios, who's getting a set with Earthquake added to its analysis so it can stop Heatran, Tyranitar or Jirachi. Or anything really that has any kind of slashes in its set analysis.
those are two really silly examples for such a serious issue. first, terrakion can barely even hurt golurk with earthquake, and golurk's not a metagame threat so ignore that...second, just because latios with eq is getting a set doesn't by any means translate to it being viable or good. i'm in ou c&c and i know for a fact we let some pretty awful stuff slide at times. also, you're making a fallacious argument here: just because terrakion and latios can run diverse movesets doesn't mean deo-d is any less broken because of it. anyways, this is simply dodging around the main point that deo-d can run specific moves to completely shut down pokemon that would otherwise wall it, and since you can't see deo-d's moveset in team preview, it's essentially a guessing game as to what it's running. ready to switch your forretress into deo-d, but afraid of hp fire? oh well, only one way to find out! long story short, deo-d's attributes of high base stats, an incredible movepool, and unrivaled versatility as a hazard setter combine to create a pokemon so definitively broken that i'm surprised it just now is getting suspected. thank you for your time, readers.