If I had to guess, they're talking about surveillance like CCTV cameras. Because you have to physically go somewhere to spend cash, your movements could be monitored. I guess it's a valid point in some places, where CCTV can be quite pervasive. But I think people overestimate the potential sometimes. Too many shows on TV where they always catch the bad guy by enhancing some blurry footage from a camera in a way that wouldn't actually be possible in the real world, heh.
At this point, I believe it's easier than ever to monitor someone through phones instead of CCTVs. I highly doubt someone is keeping a track of who uses cash at some places through
cameras - it's much easier and makes more sense to monitor someone through a device they carry around 24/7 with them.
I'd be way more afraid of an Alexa or a Google Assistant than I'd be of a surveillance camera tbh, and let's not forget the fact that a lot of people use NFC cards to pay at shops for comfort - so the ledger is already there: the bank account. With the smartphone it's as easy as the phone recognizing a customer-cashier interaction and storing the data in a server.