Rohan:Mi ne volas manĝi ion

Indeed, we can say in English "I don't want to eat something (just now)".

Chainy:Neniu ion sciis pri la maniero de ĝia pretigado.

That's a convincing example (as is Nur tiu ne eraras, kiu neniam ion faras, in the same box in PMEG, and this second one is from Zamenhof's Proverbaro).

The distinction that English usually makes between "something" and anything", that does not exist in Esperanto, as Erinja says, looks like the heart of the problem (for speakers of English).