Yes, that is true; but the geography in this manga seems to be based on Eurasia, so there should be plenty of suitable terrain.
I'm not question history, I'm questioning the shoddy worldbuilding of the author.
It's likely carts exist, just not carts you can carry on your back as well.
Smaller hand-pulled carts might also be very rare. The easy solution is to make something carried by a horse, which is inherently big, for something transported by people, if you already have the image of a big, heavy cart, you'd think about bags instead. The middle ground is always something that takes longer for someone to question. It's not fresh per-say, but it's just easy to forget about.
People are way more curious than you'd typically expect, but while I might have questioned people's curiosity a few months ago, ever since I bring a cart to do my grocery shopping, not a single time has someone not asked me where I bought it and commented this is more practical that whatever they're doing. Small reminder that in a lot of developed countries people still use bags to transport their groceries on foot, and cars for longer trips. Most people still don't use carts.
Sometimes even things that seem obvious aren't even obvious in our current world. It's like how simple the printing press is a concept, 'just make a template bruh' wasn't a thought in the minds of the people that copied books by hand for over a thousand years. Hindsight is 20/20.