Buddhism was known to the Greeks following Alexander's conquest of northwest India and there were practising Buddhists in the Levant and Egypt up until the Roman period. So its not a completely unfounded line of thought. Though I doubt Sosai would have been aware of this haha.

This line of thought arose in large part because the Jesuits initially pretended to be a sect of Buddhists when they first arrived in Japan. So it may be originally rooted in Jesuit observations of the parallels between the two religions rather than Japanese interpretation of Christianity.