I've always considered "co-witness" to indicate that the optic's red dot is co-located with the aligned sight picture, that is, your red dot is on top of your front post when the sights are aligned.

Co- means together, and witness is a reference to what you see, and I take co-witness to mean that your sighting indicators (aligned sights and red dot) are together (in the same place).

My irons and my EOTech do not co-witness, because I run a LaRue EOTech riser. Of course, I can still use my irons through the EOTech window, but my aligned sight picture is not in the same place the red dot is.

I'm not calling anyone else wrong, just explaining how I use the term.