Ive witnessed pyxies stalk and hunt as well, my Budgetts frog does the same thing. When my Fantasy Frog sees a nightcrawler, he goes for it. But for the most part, they are in fact ambush predators of the sit and wait style but that doesnt mean they are a one trick pony.
I think the notion that these frogs should be fed a diet consisting primarily of insects is a much bigger misunderstanding of their captive care. Not that they dont eat insects, but I dont believe it makes up a very large part of their diet.