The T&T authorities can (and routinely do) ask airlines for passenger details if they come across someone who has recently travelled with them, which avoids the need to ‘scan in’ when customers will likely only have their own country’s app and not all of them. There’s obviously a clear loophole here in that you could just book under the name James T Kirk and avoid detection (as BA still doesn’t ask for ID unless checking bags)... wouldn’t work for FTers though of course as you then wouldn’t collect Avios or TPs.