Just like you don't have to see it rain, to know that it has rained when everything outside is wet, Garner doesn't have to actually see them going down the stairs to know that they have left the floor and thus have gone down the stairs.
Just because everything outside is wet doesn't mean that it rained.
That may be but the timing is provided by Garner's observation that she saw Truly and Baker come up after the girls went down. No time recollection involved in that one.
The timing provided by "Garner's observation" doesn't fit. It has Baker and Truly reaching the fourth floor much later than they did.