I'm trying to think of some reason why the stress on the crankarm threads would be different; the extractor MUST push the crankarm threads away from the axle.
Proper use, of course, means the extractor is seated; but even were it not, once the screw contacts the axle and takes up any remaining thread clearance the situation is unchanged.

I wonder if the poster thinks that that stress is greater because the friction between the screw and the extractor might make the extractor turn in the crankarm (even though seated) and the helix angle of the extractor thread is thus putting extra stress on it.

Which looks like a stretch in both senses...