“These bacteria really depend on oxygen,” Professor Gieger concluded, in the same way that mitochondria do to produce energy.

Oxygen is the key to why this happened.

There hadn't been much oxygen in the atmosphere until a waste product emitted by cyanobacteria (oxygen) built up to the point that it was toxic to many then-existing organisms (which were both bacteria and what we now call 'archaea'). So an archaea teamed up with a bacteria that could use oxygen quite a bit more efficiently had a selection advantage (no idea how large the advantage was then but now it’s 3-ish (fermentation) to 32+ ATP per O2).

Current mitochondria have an electron transport chain, and a 'motor' that uses the 'pumped' protons to drive a molecular motor (ATP synthase) around an axis - the outer part physically 'jams' a phosphorus onto ADP (good video here). Gotta admit, I was a bit gob-smacked when I first saw how it works. But I’m pretty sure archaic mitochondria did not have that ‘motor’ — or at least not sticking outside their cell membranes.