Might be just stupid metrics, yes. Most guards are NOT there to "identify" anything or "arrest" anybody. They are there to have somebody watching things.
Identify is precisely what security guards are hired to do, the big issue with CCTV is unless you know when and where an event took place, you're looking through a mountain of footage for a needle. This is likely the same problem with robot guards, they cant tell when or where something untoward is happening.
Arrest, probably not as, in most civilised countries guards are instructed not to put themselves in danger. However the article mentions places in the US so that doesn't really apply.
The other thing they're there to do is deter, which a physical presence tends to do a lot better than any camera or robot. Even a cardboard cutout of a police officer has resulted in noticeable reductions in petty crime. Humans are wired to respond to human features (notably eyes) in that way. A liveried police patrol car will slow down far more drivers than 100 speed cameras.