Idk, They called it "A" First Class instead "Z" Businnes. Is there any difference? Or just airline's trick?
It is almost certainly a misinterpretation of the fare code by ITA. Aircalin doesn't have first class, so there is no way to book first class. Has happened before with ITA and multiple other airlines, happens with ITA and EI from time to time as an example, among others.