Cowitnessing can be different to different people from others examples have heard and seen stated too. If trying to cowitness and the optic blocks view of the irons? To me, it usually means need another style or type of optic or if possible raise front or rear iron sights.
To some people, cowitness is a variable of vision of the iron sight and complete vision through an optic, sometimes only partially seeing irons and sometimes just enough to aim. Am guessing that's good enough for some people? To me and likely many others ideas, true cowitnessing means completely seeing and using both sights and both at once in case of failure of one or the other.
Sometimes forgotten about? Optic adapter plates, when available, also do raise the optic from the slide, sometimes negating cowitnessing on some guns blocking original iron sights not set up for one particular optic footprint. It isn't always just stop, drop and plop any old optic to any old gun. - Wish it was just that easy every time. There's different optic footprints to consider as well as just the optic itself. - The specs of ALL the particular project are very important to go over beforehand and before buying or trying to mount any optic to verify fitment. Tying it all together can be very easy or a pain, just like trying to find an appropriate holster afterwards can be after mounting the optic on HG's.