Yeah a white background definitely makes it hard to highlight the flaws.
But, assuming the rest of the card is perfect(no holo scratch, no hidden edge whitening, etc) that one white mark should not stop it from being a 10. I’m sure you’ve all seen many different psa 10 1st edition charizards and they all have similar if not worse backs
heck, even this one that got regraded recently : www.elitefourum.com/t/psa-10-1st-edition-charizard-recased/25758/1 with a worse left corner mark got a 10. Again though we don’t know the whole situation about this card because it’s just a set of like 5 pictures against a white background lol. My whole opinion is just that if that was the only error, it should still be a 10