My Player's Handbook, in French, as well as this other source, describes Combat Style (two-weapon fighting) as follows:
Si le rôdeur choisit le combat à deux armes, il peut se battre comme s'il possédait le don Combat à deux armes, même s'il n'en remplit pas les conditions.
If the ranger chooses two-weapon fighting, he can fight as if he possessed the feat Two-Weapon Fighting, even if he doesn't fulfill its conditions.
According to this definition, logically, if the ranger can only fight as if he possessed the feat, then he doesn't actually have the feat, and thus cannot use it as a prerequisite for Two-Weapon Defense. To take the feat, the ranger would need to also get the feat Two-Weapon Fighting (which would bring no additional benefits apart from providing a prerequisite).
However, I researched a bit more and found the following (source):
If the ranger selects two-weapon combat, he is treated as having the Two-Weapon Fighting feat, even if he does not have the normal prerequisites for that feat.
According to this definition, though, the ranger is treated as having the feat in all aspects, and thus should logically be allowed to choose the feat Two-Weapon Defense, which would bestow its benefits normally (as long as the ranger is wearing a light armor or no armor at all).
The English Player's Handbook has the same description (p. 48).
Now I wonder if this is a translation difference, a small difference between book printings, or something else. Either way, what is the exact rule for this case?