I don't think you can directly correlate. How many 10000 sq km "Commercial Districts" do you know?
Ah... I know I cannot directly correlate - but that is not a problem. I underestand even the commercial center is drawn to "fill" one hex, it really represents a highly commercially-specialized town laying inside that area (even not just a town but several important trading zones). Same way I underestand a "market" that provides relevant benefits for a full civilization is not just a grocer store, but a city-sized market plaza, or several smaller specialized markets scaterred not very far away one from another in a commercial region.
It's the other way around: I underestand the market does not take all the hex, I underestand the polder does not take all the hex... is the assumption that the hex only contains the four streets of the commercial hub, the same one that assumes that there is no "space" for ships to cross the tile if you put a polder in it.
And, it is that same assumption the ones that makes me have gigantic sized archers (higher than a modern 60-store building), shooting acroos a million people city, to a single lancer whose leg is as big as the tower of the university bulidng (and that, btw, can "cross" it without destroying it). I see two full armies, one made mostly of archery units, that if takes the initiative can harass the lancer army stationed in the university region (not necessarily in the campus itself), and come back to their strategic position whitout entering in full-front combat.
There are limitations in the representation, therefore 100% realism cannot be achieved, but I don't know. For me, it feels much more real this way
Sincerely, I think there are much more difficulties to correlate the other way around.
Or, in other words:
I think with the way districts and wonders now take up entire hexes, this statement about Civ V and Civ VI hex size is untrue.
Because you are measuring down-top, assuming the represented size of the units/wonders/districts is correct... but if this is correct ¿why I am able to train units in some barracks which have barely the size of one of their feet.
The hex measurement is made top-down, assuming we are playing in a world roughly the size as ours... it's ok if you want to play 5000 years of history in a much smaller planet, but I think the "awe-inspiring" view of "leading your civilization" is we are assuming an earthly scale.