I saw a player using a razz berry despite having nanab berries

Nope.

It's the same, but razz is pre-selected and can be fed faster with less tapps on the screen

laziness or in a rush

Razz berries and Nanab berries have different mechanics. Sometimes it is good to have a Nanab berry if a pokemon is aggressive or dodgy, which happens quite often with raid bosses.
Winning raids gives you Golden Razz berries which are much better than regular Razz berries. Therefore, one can get rid of regular Razz berries but save some Nanab berries.

Nanaabs are pretty useful vs eratic raid bosses

I feed mine pinaps, because thats what they like

My team has so few gyms I never have Pokemon to feed berries to, so I just give an whatever. The only berries I don't currently toss are the golden ones, and I have 100 of those...

my money is in laziness
but aside from all reasons given above maybe he has over 100 razz and wanna get some space

Im pretty sure you can get more gym points by feeding different berries if you are feeding multiple berries to the same Pokemon

Maybe he had lots of them.

well in my case lv 34 I have over 100 raspberry's 35 pinesap's 30 or so nanab ones I get more raspberry's
on my pokastop spins so use the abundant ones first simple as that