Hi Jake,
I think the most likely explanation is that someone used a pin to see if it was glass…..and doubtless found out that it was not glass! My comments are based on having made a huge number of these eyes in a variety of materials when I used to restore pieces (netsuke, shibayama, okimono etc.) During that time I have never encountered a glass eye nor any where any type of pupil had been created by dribbling anything internally (except from the reverse as described above).
Once you get the hang of it and have the right tooling they are not that difficult to make…..just a bit fiddly - which tends to mangle your fingernails (miniature diamond drills and cutting discs on flexi drive jewellers machines).