The way these exit polls work is if you get asked and agree you get a mock voting slip, you select the candidate you voted for, fold it and put it in the box. Just like you did in the real poll. The “pollsters” then call the office and tell them the result on each slip. Perhaps they do do that periodically during the day in which case you’d be right. I’d be surprised though but if it’s secure I suppose it’s ok.
It's definitely been as soon as the polls close for years - going back to the 1980s they used to have a couple of minutes of build-up, but now it's literally the second it turns 10pm.