Painting in broad strokes here as its been over a decade since D&D v4 and WoWs high-water mark, I think people call it that because of several factors, not all of them in the game design of v4.

• It was the begining of the trends that we see today in 5e where the stock classes were designed to fit the archtype, not the style. If you played something out of the base books, it just had to work and character progression was boiled down, you had to try to brick your character.
• This was also around the time when WoW was changing its mentality to “bring the player, not the class”. This still holds up today where they design content around the players being there, not what was there. Fewer class specific shortcuts or tricks, unique traits. No one single common ability is unique to a class. For example, need temporary invuln to eat one hit that causes max damage, bring a pally, mage, or hunters.
• The migration from 3.5 to 4 left a void in class/race variety as the prestige classes took a while to convert to the new system, many were lost. Like when Disney bought Starwars and threw all the extended universe content out. D&D 3.5 had an alarmingly high number of suplimental books.
• The increase in charts in v4 felt like they could have fit more in with a PC RPG, I do realize the irony in the success of Pathfinder 1 as it just took that “charts for days” concept and ran with it.

D&D v4 to me will always be the MMO version because it was a product of its time, and also WotC scrapped pretty quickly, relativly speaking.