Right, but Darryl G pointed out that Stihl actually has run time estimates in minutes in their specs. Yes, it really does seem like the battery capacity is the limitation.
Oh, and the funny thing is that from a homeowner perspective, cordless may actually make less sense. Li-Ion batteries only last 7-10 years, whether you use them or not. All batteries eventually go bad, just on the shelf.
If you wouldn't have used enough gas to equal the up-front cost of the batteries over that much time, gas may actually be cheaper.