Azumarill is immune to Outrage and Mamoswine never directly swicthes on a dragon anyways, it just revenge kills.
+1 252+ Atk Life Orb Garchomp Earthquake vs. 252 HP / 0 Def Azumarill: 351-413 (86.88 - 102.22%) -- 93.75% chance to OHKO after Stealth Rock
Azumarill still dies to Earthquake. The point stands.
As for Mamoswine, the original comment was that Mamoswine can KO Zygarde with Icicle Crash. Yeah, I'm pretty sure that's not a revenge kill.
This was before the fairy type existed. And Flygon was instantly kicked down to UU whenever Garchomp was available in OU. Now Zygarde faces competition from both of them, and one is UU. Do the math.
Well of course Flygon was outclassed by Garchomp. Garchomp ties or beats it in every stat and has Swords Dance to boost and sweep. The only advantages Flygon had were Levitate and U-turn. Zygarde has simultaneously the special bulk of a Jirachi and more physical bulk than Hippowdon, Dragon Dance, Coil, and Extremespeed. It'll definitely face competition from Garchomp as the more offensively oriented Dragon, but if you really think it'll face legitimate competition from Flygon, you've got another thing coming.
As for the typing itself, just look at Flygon in DPP OU. It had good speed, decent Atk but with no boosting moves, and the rest of its stats were mediocre at best. Yet it still remained top tier OU because Dragon/Ground typing is just so good. As for Fairies, the only ones that resist that combination are Togekiss and Whimsicott, and honestly I can't remember ever seeing a Whimsicott during my time on PO's XY ladder.
Speed isn't the problem, it's the low base 100 attack. It's tied with Flygon for the weakest physically oriented dragon and it's slower than it to boot.
Even a base 100 Atk stat is enough to do a lot of damage. a +1 LO Adamant Zygarde has ~90% the power of a +2 Yache Berry Garchomp, which never ran into power issues, only Zygarde also now has more speed and a lot more bulk. The fact that the extra bulk can help you net a second Dragon Dance and/or survive a faster hit that might have taken out a Garchomp in the same circumstances is a nice bonus.
Mew isn't OHKO'd by Ice Beam, Moonblast and Draco Meteor, arguably the 3 best special attacks in the game now that perma-weather is gone.
Then don't keep it in on those moves. Don't try setting up on a Starmie, Sylveon, or Latios. But you're missing the point here. Jirachi and Celebi were two of the best specially bulky Pokemon in the entire BW OU metagame, and this thing matches them for special bulk while simultaneously having more physical bulk than Hippowdon. 252/252+ Zygarde is only 2HKO'd by Choice Band Terrakion's Close Combat (one of the most feared physical attacks of last generation's OU) 14% of the time even after Stealth Rock. That isn't to say that physically defensive Zygarde is its best set, but it does show how ridiculously bulky this guy is.
Lando-T drops its Attack with Intimidate and 2HKO's at worst with HP Ice. Zygarde loses.
252+ Atk Life Orb Garchomp Outrage vs. 248 HP / 172 Def Landorus-T: 212-251 (55.64 - 65.87%) -- guaranteed 2HKO
0 SpA Landorus-T Hidden Power Ice vs. 104 HP / 0 SpD Garchomp: 224-264 (58.48 - 68.92%) -- guaranteed 2HKO
Landorus-T comes in as Zygarde Dragon Dances, drops it to +0 Atk, and then Landorus-T gets 2HKO'd by Outrage while it fails to get even close to a OHKO with HP Ice. Landorus-T loses.
It doesn't have Flygon's offensive stats. In fact Flygon has the same attack and better speed. Those 5 speed points make a world of difference.
Flygon also has no way to boost its Atk bar Hone Claws, no speed boosting move, no Extremespeed, and does not have bulk rivaling some of the greatest special and physical tanks of BW OU
at the same time. Those extra speed points are literally just helping against Genesect, Hydreigon, and Haxorus, and honestly Genesect is the only one of those that I've even seen on the XY ladder (besides one SD Haxorus, which isn't revenge killing Zygarde any time soon). So you're giving up a ton of bulk, two great boosting moves, and arguably the best priority move in the game just to beat Genesect.