> Rancidification is the process of complete or incomplete autoxidation or hydrolysis of fats and oils when exposed to air, light, moisture, or bacterial action, producing short-chain aldehydes, ketones and free fatty acids.

> Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes.

So by those definitions, rancidification through enzyme-catalysed oxidation, which by proxy probably includes anything microbial, is fermentation. Vice versa aged cheese is produced through rancidification.

Doesn't sound completely different to me.

Disclaimer: A lot of the terms in those Wikipedia articles go over my head, so feel free to correct me.