Even on this Ufopaedia page, the topic is dubious, see the discussion page - nobody actually tested, in the last 4 years.
If I had to guess :
- Somebody noticed in the datafile that the item has the same flag as two-handed weapons (offset 50[0x32] in OBDATA.DAT), and conjectured that the game may take it into account.
- My opinion : If the game actually took into account free hands, veteran players would have noticed. We know this flag IS tested for shooting actions specifically (mods of the original game can tweak it) , but I highly suspect that it is not tested in the blaster launcher's "launch" action, or the stun rod's attack.