I don't really think it's that far of a stretch. To elaborate on Eusis' point:
Any action performed by the body, both voluntarily and involuntarily, is done via a signal sent by the brain. Much
like a computer's wiring and coding. If you know the 'language' it's not that hard to re-program a computer to make it think
every instance of the 'S' key being pressed is really an 'A'; despite an outside observer being able to clearly see that S was
not pressed. Considering the VLE universe has perfected Moon-Base tech, fully functional bio-android and AI tech, and cryogenic
freezing tech; it's not unreasonable that they understand the 'brain language' enough to encode artificial eyes with the proper
'programming code' that confirms the brains 'blink now' command.
In theory it's quite acceptable to believe he wouldn't even know the eye was fake even if he physically touched it. In order to reduce
phantom limb syndrome you could also encode the brain to register the 'touch sensation' to come up as 'natural flesh' rather than
'cold metal'. Again with the whole 'Schrodinger's Cat' thing; Sigma would never know the eye was fake until the box was 'opened',
via looking in a mirror or being told so by an outside source. This manipulation of sense also extents to 'depth perception'. Even if
the eye piece is protruding a bit the touch sense could also be overwritten to give a false measurement of how far out his fingers
were from his eye upon contact; since he has no visual point of reference there's only that one point of manipulated data to go off of.
And it's not like anyone would say anything about it either. Do you go up to the hook-handed man and go 'Dude...you're kinda
missing something there.'? It's that whole weird societal unwritten rule about people with mangled, missing, or fake limbs:
Don't stare, don't ask, don't even talk about it in ear shot unless that person has brought it up first and is comfortable with it.