It is theoretically possible that this encounter happened after Officer Baker and Mr Truly returned to the first floor.
However! We also have this, from the New York Herald Tribune 11/23--------------
Unlike Mr Kent Biffle's DMN report, this gives us a direct quotation from Mr Campbell.
WE raced BACK into the building: Officer Baker didn't race BACK into the building....... this must therefore refer to Depository people (at least two, one of whom being Mr Campbell).
Did Mr Biffle conflate two different Oswald encounters, one involving Officer Baker and Mr Truly, the other involving Mr Campbell and someone else (= perhaps Ms Jeraldean Reid, the person who said she was with Mr Campbell and Mr Truly watching the parade-------and who, per Ms Karen Scranton's account, talked of Mr Oswald's having come to "the door"----of the storage room?----when people were
milling and walking and so forth and her having told him someone had shot the President...?)