Mozart is incredibly difficult.
For the most part, I've noticed that the majority of pianists who call Mozart easy happen to be immature musicians, with little to no interpretation. I used to call Mozart easy until I realized the musical and technical difficulties behind it.
It's so hard to keep the left hand so soft and let the right hand get carried away, with exaggerated dynamics. Whomever said this, try the sonata k 284.... or try concerto k 466 in d minor, 1st movement.
Some of those runs are BEASTS, as well as the cadenza.
I know that famous pianists often consider Mozart the hardest if not one of the hardest composers to attempt to play.
When I was playing the 1st movement of the d minor of a youngin', even the cellist I was competing with told me how one of his relatives is a great pianist, and before one of his concerts, said "Oh ****, I'm playing Mozart".
I asked my teacher, an incredible pianists, with unmatchable feel, and for that matter, flawless technique..."What do you consider to be the most beneficial repitoire?". He simply answered, "Mozart". Mozart not only teaches technique, movement of the wrist and efficiency (you need to be light physically at all times), but also perfect phrasing abilities and ability to play as soft as possible.
I wonder how many accomplished pianists out there have not played one ounce of Mozart?
Honestly Steinway, if you call Mozart "easy", then you are not playing it at a professional level, but rather, you are only playing the notes. Don't worry, you'll learn someday.