The GP wasn't talking about complexities of Spanish and Chinese for native speakers, but for speakers of other similar languages.
Presumably, Chinese is easier to learn for Japanese speakers for the same reasons of similarity and familiarity that speakers of English have towards Spanish. If Chinese is inherently difficult, perhaps the Japanese could cope with it better than English speakers, as they had already gone through the rigors of learning Japanese, a language with similar (if not the same) level of difficulty.
