Even if you just consider the 800,000 players who finished Firelands, only a quarter of them are done with normal Dragon Soul. This means when players make comments like "Dragon Soul is easier than Firelands," they're not at all supported by the statistics. As many people had completed Firelands pre-nerf as have now completed Dragon Soul. Pre-nerf Firelands was, statistically speaking, on par with and not harder than Dragon Soul is right now.
That's not only incorrect from an analytic perspective, it's also factually incorrect. Wowprogress shows us that there were 19,500 Deathwing normal kills by January 19th, the date that the article was published. That was 52 days after the instance was opened up on November 29th. On August 18th, 52 days after Firelands was released, there had still only been 9,500 Ragnaros kills. 10,000 more guilds have killed Deathwing than killed Ragnaros in the same time span, a 105% increase. This is an indication of Dragon Soul being significantly easier than Firelands was, not "on par", as Mr. Rossi claims.
I've spoken before about how raid groups will settle onto their appropriate place on the curve of raid kills. There's essentially three factors that decide if you're going to down a boss. There's skill, gear, and commitment. Skill is the combined ability of the members of your raid to know how to play their class and role, and their ability to learn and adapt to the mechanics of the encounter. Gear is just that, the quality of the gear that your raid has. This provides a buffer, stronger tanks are less likely to die, bigger healing throughput keeps the raid up, and more damage output shortens encounters. More gear means an easier encounter, which means less skill is required. Then there is commitment. All other things being equal, a guild that raids five days a week will progress further than a guild that raids five hours a week like Legacy. Skill x Gear x Commitment = the timeline upon which a group can expect to down bosses, assuming the difficulty is equivalent.
Many people measure the objective difficulty of encounters via commitment required to down the boss, either in number of pulls, or number of weeks spent learning a particular encounter. The reason for this is that within a raid group, skill and gear tend to be equivalent at equivalent points of progression in different tiers. A guild in 346 gear working on Halfus will take about the same amount of time as a guild in 359 gear working on Shannox. It's a rare occasion where Mr. McEatstheFloor suddenly becomes an amazing player. These things rarely change.
A guild that requires three months of farming gear in order to raise their SkillxGearxCommitment quotient high enough to down the final boss in the instance does not magically become skilled enough to clear the next instance on the first day, or in the first week, or in the first month. If you were not in the first 10K Ragnaros kills in T12, then you shouldn't expect to be done with normal mode Dragon Soul yet. However, 10k players beyond reasonable expectation have already cleared the instance. We can use this to establish just how much easier Dragon Soul is than Firelands, and through that assessment, we can build reasonable expectations for when a raid group that cleared Firelands should expect to be able to down Deathwing.
The 19,500th Ragnaros kill didn't occur until September 26th, 91 days after release. There's a 39 day disparity between hitting that benchmark in Dragon Soul, and hitting it in Firelands. You can either make the assumption that the lack of difficulty simply frontloaded guilds, that the guilds that downed Rag in the first month downed him the first week, and then things settled. You can also look at it as a compressed schedule, where every day spent in Firelands is equal to 60% of a day in Dragon Soul. It's more likely a combination of the two. However, this shows that it's reasonable to expect your group to reach a milestone in DS about three to five weeks earlier than it took them to hit that level in Firelands. It is not reasonable to expect kills to be rolling in three to five months sooner.