The iOS MAC randomization wouldn't help here
What that does is use a random MAC address (in a particular range, so it won't collide with 'real' MAC addresses assigned to Apple devices or anyone else's) when a device is looking for access points to associate with. If it finds one it has associated with in the past, or you tell it to connect, it uses its real MAC address from that point on. This is intended to prevent tracking of individuals via their phone (they might not have your name, but they know when you come back, and with enough visits that leave other evidence like purchases they could tie your name to your phone's MAC)
Unless this cop's device is simply masquerading as a passive access point and hoping things will associate with it, it should find stolen iOS devices same as anything else. It sounds like he's actively sniffing, like the Google Streetview car, but rather than sniffing the data like Google, he's (for now) just sniffing the MAC addresses. Encryption won't help there, since that's sent in the clear.
This all seems rather pointless, and is probably a foot in the door for doing more widespread sniffing of wifi traffic as they drive by like Google was, except on an ongoing basis as cops drive around town 24x7. As if cops really care about finding someone's stolen Dell laptop that was $500 when it was new in 2012 and is worth $40 today. Stolen cell phones are already mostly a non factor in the US these days since most phones are new enough to have Activation Lock on iOS and the equivalent on Android.