Worth noting that for many of those people, their frame of reference was built off A Certain Private Server's* tuning, which said server's devs openly admitted was overtuned to keep their server challenging for experienced Vanilla private server players as well as to provide a tougher experience to bite into for players alienated by retail's easier level grind. There was a lot of guesswork involved on their end, too, as they had little if any of the server-side information to work with, and they'd always err on the side of making it a little bit harder.
Classic, on the flip side, is balanced around Blizzard's internal 1.12 client, which they refer to on a semi-regular basis if people mention something feeling off even though it matches up with the internal client. So for players used to A Certain Private Server's core (which quickly became the go-to core for Vanilla private servers after its devs released the core), Classic is noticeably easier, and strats that were developed in response to the increased difficulty in that server are crushing the historically-accurate 1.12 Classic difficulty balance.
* Note: I know typically discussing private servers is a no-no, and by no means am I attempting to promote their use. However, context is important in discussions like this and many of the Classic guilds crushing raids on release day have done so using strategies and experience cultivated on those servers. This context is important to keep in mind when many posters were discussing Classic's hypothetical difficulty, as even unintentionally, it can color one's memories of the original content. Even for those who never played on one, the experiences of those who did and their rhetoric dovetailing nicely with their memories of a more difficult experience (typically due to inexperience with the game) can form a sort of Mandela Effect.
What we're seeing now is just how much those private servers overtuned their content since they also used 1.12 as the building patch for balance and class design.