My stance is born from experience in both tool handles and bows. I know for a fact that the sap wood is superior. I'm also in some pretty good company with that stance.
Ideal or even acceptable characteristics for bow wood do not completely equal those for axe handle wood.
Bows exhibit somewhat different type of shock than axe handles.
Your "knowing for fact" means you trust your own personal experience over multiple independent studies.
Sharing your own experience is fine, and while it doesn't have to agree with the views of others, it will be more believable if you could share links to studies contradicting the ones FortyTwoBlades provided above.
Also, calling FortyTwoBlades' post trolling without providing evidence it is actually a trolling, is just a cheap ad hominem argument.
I am curious if you could provide some support to your stance beyond your own persona; experience. You might be right, but it needs to be substantiated with independent studies and no name calling.
As for the original post, in the past, when hickory was more abundant, axe handle makers likely picked species with higher average strength properties and preferred wood from second growth (younger) trees. Handle wood was likely air dried, which is less harsh than kiln drying.
Nowadays, it is more difficult to know what species are used and also handle wood is frequently kiln dried. Kiln drying might not be always optimized for all the variable (by species and age) wood dried in the kiln.
That might contribute to small splits which eventually lead to handle failure.
Old Axeman brings up an interesting issue, that of a handle consisting of both sapwood and heartwood. This is a common occurrence nowadays, and I don't know if the studies quoted above had tested mixed sap/heartwood specimens too, or only clean sapwood and clean heartwood ones.
When I look at the handles in stores, I frequently see mixed sap/heartwood, knots, checks, seemingly too dry wood, and have seen too many handles which have already split while still in the stores.