The CEO claimed it's because the USB C port can withstand more than the 3.5mm so it's more sustainable to use that. I honestly don't buy that argument because now you're putting all of that load from 2 ports into 1. Also, if they were truly Fairphone, I should be able to buy a replacement headphone jack if it does break.
I think he eventually admitted it was a mistake, but it turned from a definite purchase to a zero for me.