As pointed out by the OP; you can build Mechanized Infantry without bronze working, iron working, or metal casting.
Good luck constructing and powering such a vehicle with only stone & wood.
Hah! Re-read the OP... you don't even need the Wheel-tech to beeline mechanized inf.
So I guess Fred and Barney would have to put some sort of skies under their car?
That has got to be the silliest discrepancy in the tech tree: Being able to build a unit that relies on wheeled vehicles to move around - without having to have researched the Wheel.
I didn't check how easy it is to beeline other stuff, but I did have a game the other day where Victoria had ironclads in the year 1000 AD - while my science-crazy Civ was about to get into Square Rigging. That was on "King", IIRC, so science-boosts for the AI weren't crazy high.
One of my main gripes with the tech-tree:
How is it that Archery is a dead-end? Surely, in real life, that tech lead to more advanced objects, concepts and techs, probably eventually even to stuff like ballistics or firearms. Or is the game trying to say that a stone-age bow is the same as a medieval English longbow? Or as a Hunnic composite bow?
Plus I really hate the fact that the gaps/jumps from unit to unit are wide enough to drive an aircraft carrier through them. Going from swordsman straight to musket, then straight to modern infantry. Or (even worse) straight from pikemen to AT-teams. Translated into the real world this would mean that WW1 or the US civil war were fought with pikes.
The game needs things like:
Composite bowmen (between archers and crossbowmen)
Riflemen (basically an 18/19th century gunpowder unit before WW1 infantry-tech hit)
Lancer/anti-cav unit to fill the gap between pikemen and AT-teams
Late swordsman or Maceman (equivalent to the longswordsman of Civ V, pre-gunpowder-unit but still way stronger than the early swordsman).
The funny bit is that they did include three eras of air-combat tech (biplanes to modern jets), when that stuff usually doesn't play as big a role in Civ-games (at least not in mine) as early/mid game melee and ranged units. And at the same time they've made building up an airforce quite a bit trickier than before, thanks to "Aerodrome"-districts.
S.