No, that's just as parenthetical, which changes the meaning of the sentence: if read while skipping that part, you get a totally different [and false] statement:

> The reason that `fields_for` constructed a form is that it won't work for any form nested deeper than one level deep.

As just saying "no" doesn't help very much, here's a suggestion:

> NOTE: `fields_for` constructed a form that produces a hash, instead of an array, because an array would not work when nested more than one level deep.

I dislike the suggestion that fields_for is constructing a form at all -- it's only defining some fields. But I won't suggest changing it without properly reading the whole guide for context.

More importantly, did I maintain the meaning of the original sentence? I think I did, but I'm genuinely not sure.