That would incorrect. vBulletin 4 can be setup to allow attachments.... Although I can't imagine anyone wanting to allow "guest" to post attachments. That just comes off as an easy door to abuse to me.vB doesn't allow guest attachments either (unless you go back to vB2).
Anything is possible.... I've not really played with vBulletin in a long time now (or at least it seems long enough).Maybe it changed in a recent version. I checked this on my vB4 test forum and the guest group doesn't even have attachment permissions.