I don't think it's that the elderly are "dumb enough" to believe that their robot companion is human... it's that there may be some benefit of even that artificial simulation of companionship compared with having none at all.

I'm sure they know they're not talking to a human, but maybe, even on a tiny subconscious level, if they get even a fraction of that companionship in a simulated way, it is presumably better than the alternative.

Recounting memories from your youth with a robot is not as enjoyable as sharing them with real friends, but maybe it's slightly more engaging than being stuck in a nursing home bed all day by yourself watching TV.