Most religions and belief systems did not originate from the handful of figures the Baha'i writings identify as Manifestations of God, Indigenous religions for example. From a Baha'i perspective they are not excluded or invalided because they are not specifically mentioned. Hinduism is vast and the Vaishnavites are not the only valid group because they worship Krishna.Would it be safe to say, then, that the Baha'is include only religions that honor any of those afore mentioned figures?
The Quran (Surah 16:36) states there is Messenger for every people so this idea of a Creator God guiding every peoples is nothing new. Religious figures such as Buddha, Jesus and Muhammad built on what has come before. With change comes affirmation of what was working well in the past but also recognizing some practices and beliefs need to be discarded.