On HFC release you entered the raid with T17, Fury was superior on AOE and Single Target (because of Arms T17 4 set nerf), Arms was still superior on cleave. The "difficult" fights were Tyrant, Xhul'horac and Archimonde pre Mythic and all of those favoured Arms from the beginning, Fury was fine on Xhul but most likely you would play Arms for add cleaving (execute + SS), Fury gained real strength on that fight only with the 4x T18 (but then so did Arms). Most decent raiding guilds were handing Warriors tier because Arms was overpowered with their T18 in the early weeks meaning most raiders already had 4x T18 by Archimonde Heroic (by Archi normal if you were lucky).
Arms was not more gear dependant than Fury, if anything at the start of WOD it was the opposite with Fury being super reliant on having high crit to even function, whilst Arms was performing extremely well in Highmaul and only became set bonus reliant later with tuning changes + scaling of other classes, it was one of the most dominant specs in Highmaul and super strong well into Blackrock Foundry. Lets not forget that in BRF Arms still had the use of Bladestorm because mastery scaling had not yet settled in, so Arms had strong single target along with superior cleave and AOE that could compete with Fury on most fights.
You're talking about Arms pulling 30k at ilvl700 but I was pulling more than that in the ilvl 670s! Hell on my first kill of Gruul HC in the first week of BRF I pulled 31k dps as Gladiator, the first week it released. Here is proof.
https://www.youtube.com/watch?v=d49sgLJbNhA - 31k as Glad first week of BRF.
https://www.youtube.com/watch?v=6TRpAKZ1Uzs - 34k as Arms at ilvl679 just after Arms got nerfed, less than 2 weeks later.
https://www.youtube.com/watch?v=bdIF4Z5PEd0 - 43k on Twin Ogron during Highmaul.
These numbers you speak of don't add up, and I was not in a top guild at the time, this was a semi-casual guild of friends. Had I been raiding in a higher guild like the last year or so those numbers would have been higher due to faster kills, and I'm sure many top raiding warriors around here can post logs/videos pointing out just that.. Arms being behind Fury is some sort of figment of your imagination, both specs needed set bonuses painfully but prior to HFC it was Fury that suffered that more so, and with HFC having so many important cleave fights (Archimonde key among them) Arms was the go-to choice from the beginning.
https://www.youtube.com/watch?v=orBHTTeispE - Arms in T17 set on HC Xhul, prior to me getting my T18 set bonuses (or Archi trinket, obviously). I was not even at ilvl 700 in this, probably around 695 with T17 bonuses. Notice how the non T18 Warriors dominate because it's a cleave fight with adds to execute? Arms was strong for HFC progress before you even got T18. See the guy who finished 3rd out of of the 3 Warriors? He was Fury, an exceptional player but Fury.