Isn't 1 c. light brown, 1 c. white, & 3 T. molasses the same thing as 2 c. dark brown sugar?

I'm sure that you could certainly use 2 cups of dark brown sugar, and have a lovely pie. The conundrum of molasses might relate to the fact that there are various types of the ingredient- from light, dark, blackstrap, and treacle, and including sorghum if you wish. Plus we get into sulphured vs unsulphured. Since sweetness decreases as color darkens, and bitterness increases as color darkens, you could influence your final pie flavor somewhat by your choice of molasses too. The subject gets a bit murky if you consider what sort Julia Child's aunt might have used as well. That would take us back into the early 1900's, and molasses was produced for many reasons other than cooking. Like making munitions, for a start. Your great grandmother would have adapted her recipes to use the sort most easily available and familiar to her. Which could explain the odd mixture called for in the recipe.

As an aside.... I have noticed a great discrepancy in the "darkness" of different brands of dark brown sugar. Have to wonder if there is a difference in the amount of molasses added back in to those different brands.

The relationship between white sugar/molasses and brown sugar is slightly nuanced because of the kind of commercial molasses used (for a longer explanation check out https://www.seriouseats.com/how-to-make-brown-sugar-from-scratch)... but for all practical purposes, most recipes use 1T molasses per cup of sugar for light brown and 2T for dark brown, so using all dark brown sugar (~4T molasses) vs 1 cup light brown sugar (~1T molasses) plus 3T molasses should work just fine. I wonder if it's a peculiarity of what was available in 1982/whenever Julia's Aunt Helen was writing it.