While it is true that pedals will tend to self-tighten during use there's still a window of opportunity where a too loose assembly will allow the pedal to wiggle back & forth and chew up the threads that way. What do the crank arms look like? Such a radical shortening might mean that there isn't enough metal to cut threads in at that position.

And no, normal aluminium arms do not use any inserts. Fofr the overwhelming majority they're still strong enough.