I’ve never come across any zone-specific research on mitochondrial response either (i.e., Zone X vs Zone Y, etc.).
My guess is that, being largely a human construct, the various “zone” definitions are not overly discreet - they’re vague descriptions across a continuum. As such, drawing overly specific conclusions about which “zone” produces the most response from mitochondria is much ado about nothing.
It’s pretty clear that exercise is good for your general health. Vigorous strength exercise and endurance aerobic exercise are both important aspects of challenging one’s body.
But the hormesis principle always applies. There’s an essential balance between too much and not enough - and this is likely the case with respect to promoting mitochondrial health.