I think it's a case of the GA trying to save time and cut corners in every possible way to assure (read nowdays: "shoot for") an on time departure in light of the understaffed conditions. The GA could finish up paperwork and close out the flight documentation while you got resettled in your FC seat. By the time s/he came down with the pax manifest, s/he could simply close the door, and would not have to wait for you to transfer luggage, take your seat, etc. In the case of some pax, those last minute onboard upgrades with an existing seat exchange take 3-4 minutes, even with certain nimble, concourse-runner Silver Elites.
I doubt that the WP number meant much of anything to that pax-courier as it would to many of the regulars on FT.
Those GAs are under so much pressure with few supports. It was not ideal to have another pax deliver your upgrade to you, although I believe that GA's only intention was to serve you efficiently while striving for an on time departure.