Yes, thin, weak walls can be due to going too long between trims, among other reasons, as anything which compromises the health of the hoof tends to compromise it's growth & strength. Especially when shod, as there is a lot of extra load on the walls and circulation can also be inhibited. I'd be inclined to think of this as a symptom of needing a break from shoes, among other things.
Good to see those pics Tiffr, showing that your horse has obviously been well managed for that crack to grow out so well. Wow, her feet must grow incredibly quickly for it to have been gone in 6 months!
However, I suspect you're discounting what are likely the most important factors - the way she's trimmed & managed and have attributed the 'cure' to something that evidence suggests is less likely. While diet & nutrition is a very important & supplements such as you mention can indeed have good effect, research suggests you don't start seeing any evidence in the feet from changed nutrition for 6-9 months. Topical goops have also been shown to have virtually no positive effect on health and can actually have negative effects. Lost my bookmarks, but perhaps Puck or someone can give you a link to a good study on hoof dressings.
my understanding is that laminae (the main ingredient that makes up your horse's hooves) is a lot like stem cells. It goes where it needs to go.
Never thought of them as stem cells, but I really like that description!