I think what they mean is that it assumes that the demand curve, uh, I guess goes off to infinity (or just, some extremely large number which can be practically modeled as "infinity") near the current price,
meaning, that you could sell arbitrarily large amounts of it without substantially decreasing the price that people are willing to pay for it.
Which, yeah, that assumption seems false in practically relevant ways. If you sell a whole lot of [cryptotoken X], the price is likely to go down?