Just speculation of course, but I think the phrase reveals Eleanor's understanding that Earhart's strength was not as an aviator but as a symbol and spokesperson for aviation and equality for women.
I think I would call that an educated assessment, not a pure speculation. I think her weaknesses as a pilot caught up with her finally. Not necessarily her physical control of an airplane, although she doesn't seem to be an ace at that either, but her overall competence, including preparation, planning, taking charge of a risky endeavour, maybe you would call it executive ability. In other words, if I owned an airline, would I hire her to fly my flagship route? Knowing her record, I doubt it.
Yes, maybe this is uncalled for, and a unnecessary negative opinion, well of course it's unnecessary. I have to give her credit for being larger than life, symbolizing all sorts of good qualities, for doing the right things at the right time, for making herself into a heroine. For giving us all something to mess up our minds with 75 years later.