Yes, that’s not the first time the programme has shown a similar issue, also without explaining how it’s possible and why TfL doesn’t just block the card. Though the programme is interesting, it’s very low on detail. Perhaps that’s intentional to avoid giving away trade secrets but it does rather spoil things for anyone who has the slightest idea how the system works.
As I’ve said to TfL on more than one occasion, stopping evaders blatantly pushing the gates open would yield much better results in reducing fare evasion. I think the programme backs that up: the hours spent by the staff shown waiting at stations for one person would be better spent just asking the card company for the customer’s address, and employing burly SIA-accredited staff at the gatelines.