The Baha'i Faith is not mythical.
The people who make up the religion are not myth, of course. Any claimed entity by the religion certainly is, however. A little deeper dive on what myth is can be found on Wikipedia. Keep in mind however, the whole idea of acknowledging and highlighting the fictional foundations of most religions creates controversy, so there is tendency to soften definitions. At it's core, a myth is simply a fiction held by someone to be true or non-fictitious.
Here are some quotes from Wiki:
"Scholars in other fields use the term "myth" in varied ways. In a broad sense, the word can refer to any traditional story, popular misconception or imaginary entity.
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In colloquial use, "myth" can also be used of a collectively held belief that has no basis in fact, or any false story. This usage, which is often pejorative, arose from labelling the religious myths and beliefs of other cultures as incorrect, but it has spread to cover non-religious beliefs as well.
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Since "myth" is popularly used to describe stories that are not objectively true, the identification of a narrative as a myth can be highly controversial. Many religious adherents believe that the narratives told in their respective religious traditions are historical without question, and so object to their identification as myths while labelling traditional narratives from other religions as such."
Quick search on Baha'i website gives me this:
"Bahá’ís consider the Báb to be both an independent Messenger of God and the forerunner of Bahá’u’lláh (“the Glory of God” in Arabic), who is the founder of the Bahá’í Faith."
So, here, the referenced entity "God" is the fictional entity held to be real by those of the Baha'i faith. Given the definition of myth, it seems safe to characterize Baha'i faith as a myth based religion, just as most other religions are.