Databook never stated it only works for low level genjutsu I dare you to cite that.. It stated that what it's capable of reversing depends on the user and his sharingan.. And since itachi is a master of master of genjutsu's how would this be a problem?
One decrypts a genjutsu that has been
applied onto them, and casts back a jutsu
with identical effects onto the opponent!! It's
a form of illusion reversal, but in order to
instantly read through a jutsu and cast it in
return, the Sharingan's power is necessary.
As genjutsu is initially used to confuse the
enemy, this technique which sends it right
back to its caster deals a high amount of
mental damage, more than anything else.
Every half-baked genjutsu will fall prey to the
mirror that is Sharingan!!
Caption
-The mechanics of a false image is duplicated
by these eyes, its effects largely turned back
onto the enemy!!
Picture comments
-Conditions such as the presence or not of a
Kekkei Genkai and the
relative strength of
both casters play a great part when operating
this jutsu.
-Thanks to the Sharingan's power, Itachi uses
a similar technique.
