The whole point of the barcodes is so they can be checked/read via the computer to stop multiple persons on the same boarding pass going airside. I'm not sure whether the airport has access to flight manifestos- or whether the barcode just contains all the information. I'd assume the latter.
Generally, normal boarding passes are not scanned. But A4 ones should be.
As a side interest, anyone that wishes to replace A4 boarding passes with legacy style ones can usually do so if you state the A4 passes wont scan.