I just get my Affinity Photo today, and the first issue I have with the software is also this! (Resize feature under Document using dpi, instead of ppi) I search posts relating to the topic on the forum, and it goes way back to 2016. I'm so surprised they still didn't correct this. and keep using the wrong term in 2021.
Even more surprises, the response around the same issue from a certified moderator in one of the old post goes:
"..., but there is a widespread acceptance of the term DPI as being understood to mean PPI. "
No, it's NOT a widespread acceptance, it's being WIDELY MISUSED.
I've seen many comments on loving how affinity actively keeps improving its product, esp in response to user feedback. But to keep misusing dpi vs. ppi on more than one place is BAD! Plz make corrections ASAP, it's a huge shame that this kind of mistake would even appear.