While the rules of flying don't mention Lifting and Dragging I do feel we need to apply some "D&D-rule based common sense". Normally flying has a limit of Light Load, which has a weight range. Lifting and Dragging start at weights above the Light Load weight range. So Light Load is the limiting factor for carrying while flying and you can't fly while Lifting or Dragging.
Similarly, Medium and Heavy Load have weight ranges that are lower than what a character can lift or drag. Which is also why Lifting and Dragging have heavier movement penalties than Medium and Heavy Load.
However;
We don't need extreme scenario's, the
Overland Flight has an exception of the fly rule that allows heavy load flying, we known V has that spell. We don't see Celia carrying people all the time [citation needed]. So she could easily prepared this for a special night, rather than this is something she can do every day, every time.
What I am trying to show, with help from those before me, is the minimum strength required for Celia to carry Roy. Because you can fly with a heavy load (regardless of the source), we can set a minimum of strength required to carry the lightest version of Roy. Hence the 14+ strength.
Roy might be heavier, carrying more than I assume or Celia isn't using magic. Then Celia needs a even higher strength, but in general people find that more unreasonable.
I say it's impossible to explain Celia's ability to fly while carrying Roy with 13 or less strength within the D&D rules.
P.S.
I do agree that seems more likely, but this thread is to prove things within the D&D rules. Not to prove which rules are being used and which ones are not.