There are two tricky things about learning a new language: first, the other language is not your native language in disguise! You have to learn that other languages really do operate under different rules. This takes a while to 'get', but once you do you can adapt easily to learning the basics of additional languages.

The second tricky thing is irregularities - exceptions to the rules. Nonstandard verb conjugations, idiomatic phrases, irregular spelling, things like that. (In French for example, regular 'rendre' -> 'je rends', but irregular 'être' -> 'je suis'. Sacre bleu!) You'll probably notice that children learning their native tongue get irregular forms mixed up for years after they learned the basic rules - "it breaked" instead of "it broke."

When you're learning a foreign language for the first time, you generally have to deal with both of these tricky things simultaneously - many irregular word forms occur in the most frequently used words (ie, être 'to be' and avoir 'to have' in French), so must be learned immediately before you can even hope to communicate in the language.

Planned languages such as Esperanto are generally highly regular, so tricky task #2 is minimized - you only have to learn how verbs work once instead of three regular conjugations plus a couple dozen irregular verbs. So, you're free to concentrate on task #1, understanding where your assumptions about language are not universal, and learning to adapt your brain to deal with another language.

The FAQ's vague mention of this is derived from actual studies performed in England and Sweden, in which students tought a year of Esperanto followed by two years of French (in the English study) or German (in the Swedish) were found to be more conversant in French/German than the students in control groups who had taken the regular language for the same amount of time. I don't have a citation handy, unfortunately, I'll have to see if I can dig out the book that mentioned them in more detail and check the bibliography.