Yes but if you think in base 8, then you literally wouldn't know what 12 or 9 are and how much to add together. Obviously you'd have the concept of those numbers, but they'd be written differently. 12+9=21 in base 10 would look like 14+11=25 in base 8.
counting to 20 in base 8 would look like this. 1,2,3,4,5,6,7,8,10,11,12,13,14,15,16,17,18,20,21,22,23,24.
So if you weren't at all familiar with base10, it's conceivable that you'd have trouble making the conversion on the fly. It's awkward but totally possible to go down from 10 to 8 for me because it's just a matter of removing a few numbers. But what if you've literally never heard of the number nine and don't know it's value. And to you, the number 12= 10.
That being said I don't know if that's even the actual reason for her having trouble in the comic but would very much like the sauce because that cat is cute.