Barb (TV survey) privacy?

So my mum uses BARB (the service where they send you a device that connects to your TV and you select who is watching and it monitors what you watch) in return for vouches.

I came home from travelling and bought a new smart tv for my bedroom and they called my mum and asked to come and set up the device for my TV.

My question is - how did they know? The only thing I can think of if that it knows what devices are connected to the WIFI router which seems like a bit of a privacy concern to me.

I talked to my mum and said I do t want it installed as I don’t want it cluttering up my setup and the annoyance of having another remote and it bugging you when you turn on your tv numerous times a day.

Also, I don’t watch any form of live tv, player or ITVX - just YouTube and occasionally Disney Plus and Netflix - I’m not sure if this is even monitored?