But they looked so genuinely surprised they couldn't get through
- almost as though they weren't "acting". Anyone know?

It could have been a setup but may be because TfL staff
passes are only valid on tubes buses and DLR, not NR. I
expect that's the sort of pass that they're issued with?

If it was Liverpool St apparently staff passes do open those gates as
part of that route is available to TfL staff (like on FCC between
Moorgate to Finsbury Park is available on a staff pass).

D