I've read that theory, too, but ဂ is written with just one stroke and if you look at palm-leaf manuscripts in general https://en.wikipedia.org/wiki/Palm-leaf_manuscript there are plenty of angled shapes. (Ironically, the example Wikipedia uses for Burmese has a more square style instead of the now-dominant curves.) So I wonder whether it's all just a post-hoc rationalization for historical happenstance. It would be interesting to know whether e.g. Sundanese ᮕ is similarly written with two strokes.