Is it legal for a security guard/non-sworn officer to cite a driver for a registration violations in CA?

Generally no, but it depends upon the arrangement that the city has. I know of one California city that uses a private security company to cite vehicles that are blocking the street sweepers. The key is the citation itself. If it has the name of a government agency, then they have been contracted. It may just have the name of the company and be more of a courtesy notice than actual citation.

I would imagine that they are doing what I do every day of the year, as I walk to the shops or pub I photograph cars with an out of date tax disc and send it to the authorities, collared over 120 by now.

PS I'm retired.

Some state or local employees can and do write out citations for such things as improper registrationor povertime parking.

In my city, watch your parking meter or the meter maids will write you a ticket.