Two things...
"Lots of the sutras of Mahayana are attributed to other people, rather than Siddy. But again, who wrote them doesn't ultimately matter, it is the words that matter. "
"Siddy had to exist because the Bahai story declares him to be a messenger of the abrahamic deity. "
He not only had to have existed, but he has to have taught about the Abrahamic God. And, because he is a manifestation of this God, his teachings alone are the truth from that God.
Just knowing the very basics of the Buddha story, I don't see how it fits into the Baha'i interpretation. Yet, for Baha'is, it has to fit. That is what the argument is... are the Baha'i true and correct? If so, then the truth about what Buddhism is and what the Buddha taught are what the Baha'i Faith says he taught. And that means for hundreds of years, Buddhists have been believing and teaching the wrong things.
That's incredibly important... Because if the Baha'is are right, Buddhists, Jews, Christians, Hindus, Muslims and the others are partially wrong or maybe even completely wrong. That puts a lot of proving to do for the Baha'is, if they expect others to believe that... And they can't.
They can get liberal and nominal believers in the other religions to go along with them, but it's not like those people had a very deep commitment into their old beliefs. For them, the Baha'i Faith is no doubt a wonderful religion. They join and do what? Keep practicing and believing in their old religion? No, they believe and practice the teachings of the Baha'i Faith.
And it's great for most of them. Some do drop out, but many stay. So, what's the problem with that? Baha'is are told to go "teach" the Faith. Tell others the Promised One has come. And to tell the people that we are heading for disaster, and our only hope is to unite and work together... Then in the fine print it says... that to truly solve the problems of the world, we must believe and trust in the teachings of the Baha'i Faith. Nothing else is going to save us. No other messenger is coming... not for several hundred more years.
They say the Christ, Maitreya, Kalki, the Mahdi has already come. All in the same person, their prophet. Is it true? Is there proof? Is there even any reasonable evidence to believe them? They think so. But they've already told us what that evidence is. And some of us aren't all that impressed. And we doubt and question it. They don't have anything else to offer... except time will tell.
Since I first heard of them, fifty years ago, I think in some ways they've gone backwards. Back then there was talk of the "lessor" peace happening by the year two thousand. They believed the Baha'i Faith was going to finally become recognized as a major religion and come out of "obscurity". Baha'is were going out on what they called "mass teaching" projects to make more converts. In the 80's they put out their "Promise of Peace" statement and had a big peace conference in San Francisco. What happened?