I don't feel all that strongly about this, so I'm not sure why I'm belaboring this. But I'm not certain that we should be just basing this on determining minimum strength. Obviously that is the goal, but if minimum strength requires such a convoluted explanation to even work, then I don't know that it makes sense to list it.
For 14 strength to even work as an explanation, it seems as though we have to assume off-panel casting or magic items, and while it's possible that Celia has Fly as a spell, that's such a radically weird thing for her to take that I think it would require some further evidence (plus, if she has Fly, she can cast it on Roy, too, and not need to have 14 strength at all). If there's off-panel casting of Fly, then why not also off-panel casting of Bull's Strength (and then she only needs 10 strength), or Roy weighing 150 lbs and Strength 13 (or 9 and Bull's Strength).
I'm not sure any of this is a better fit (by thread rules) than a house rule of some sort (and once it's house rule, it could be either no carrying capacity rules at all for flying, or it could be that you can "lift and drag" while flying with a 5' move speed, either of which would match the on-panel evidence just fine). Am I missing a simple way that she can carry a heavy load while flying that would justify strength 14?