I think, the fact that Buddha is believed that was enlightend under the Tree, can say, that He was enlightened by some greater being(s).
I have never heard any reference to that supposed greater being.
The idea doesn't fit at all well with what is actually in the Tipitaka, either.
You could claim that it is not specifically denied, but really, that is grasping at straws.
I thought that among Buddhists, there is at least a view, that believes in Oneness of God.
Again, that is not specifically forbidden. But I am not aware of true concepts of God in Buddhism.
Even the Hinduist deities that we inherited are basically said to not be true deities, and in fact to be at a handicap of sorts compared to human beings.
Anyways, regarding that, this is what Abdulbaha said:
"The founder of Buddhism was a wonderful soul. He established the Oneness of God, but later the original principles of His doctrines gradually disappeared, and ignorant customs and ceremonials arose and increased until they finally ended in the worship of statues and images." Some Answered Questions
That sure reminds one of the Muslim opinion about Christianity and Judaism: it used to be correct, but lost its way, so follow us instead.
I guess everyone is free to believe in that, but really, it is much like wanting to have the cake and eat it too.
Sometimes I wonder why the Bahai Faith claims influence of the Dharmic religions. It's very Abrahamic approach to religion really makes one wonder if it even understands what Dharma is.