Any opinions on why my plant is so tall?

By the stretching and the large internodal distances, I would say your first thought is correct, the light is not strong enough. Simple.

I grew an autoflower (Tangerine Dream) that was supposed to only be about 30" tall... grew to over 50" and was a pain in the neck! Your plant is NOT stretching because the lights are so far away, truly... I have friends who normally keep their lights about 4' away from their grow and the plants get to be the height they get to be - often, with their autoflowers, not much more than 23-26" ... No, what you're dealing with is simple genetics and it's nothing to cry about! You'll have an amazing harvest. You CAN gently pull the top flowers away from the light if it looks like they're getting burned but I've got a grow now where the main cola is literally sitting 1.5" away from the light and is as happy as a clam... no burning at all... Still, you can do some light LST by pulling them away - it won't hurt and it would open up the lower bud sites to more light. With my Tangerine Dream, since it was threatening to rip the top of my tent off, I was doing LST right up until harvest and one of my LST attempts was actually HST in that I bent the poor stem into a 90-degree angle that I hadn't meant to do.... didn't bother the lady... actually helped with the lower buds! Good luck - it's all a grand adventure and your grow looks great!

Plants stretch for three reasons. Eighter the lightsource is far away, you have lots of heat or just genetics. When looking at your diary your lights are way to high probily because of your long stretched main cola. Just bend your main cola with dread carefully and lower your lamps, Goodluck Growing :)