I've just try it myself, and indeed it work, but it shouldn't.
Several report have been made about problem with operator and in the version.txt of mirc 6.3 :
39.Fixed /if parser not reporting invalid operator for => and =<, and added not support for !> !<= etc.
So !> !>= !< and !<= should report an error. It looks like it has been already fixed but it's not the case.
The = sign, in real, is only needed for older version of mirc (6.21 and prior, i think) but should always be used because it's the right syntax.On an older version, the line with var %listops would display an error.