Seems they are fishing to get customers off other ISPs onto their service for this trial.
Not fishing, past tense.
The trial was open to applicants many months ago via the BT Trials community. It stopped taking applicants a while ago.
They were looking for anyone with a single Openreach FTTP connection that was out of contract (with any provider).
I'm still a BT trialist from about 6 years ago when I was actually with BT.
I've done 2 EE mobile trials in that time but wasn't picked for this particular trial unfortunately.