Handaxes are Part of the Extended Mind 2

A second reason Malafouris, 2010, gives for believing in EC is:

For another, the more we study material culture, the more it looks like a genuine element of the human cognitive system and its long-term development. Thus, given the archaeological preoccupation with long-term processes and the study of material culture, one would expect some questioning of the conventional boundaries and intuitions about the whereabouts of the human mind.  (Malafouris, 2010, p. 15).

But, doesn't this just beg the question?  Why is it that the more one studies material culture, the more it looks like a genuine element of the human cognitive system?

I'm fine with the claim that material culture is an element in the long-term development of the human cognitive system, because that is perfectly consistent with HEMC.  But, why think that because some factor is an element in the long-term development of the cognitive system that it is part of the cognitive system?  Maybe a regular night-day cycle of light is an important element in the long-term development of the cognitive system.  That does not seem to me to make the cycle part of the cognitive system, rather than a causal influence on the cognitive system.