Yes, Petr Baudis decided on that name (and also "upstream"). [1]
He regrets it now because of his lack of English language knowledge at the time and that "main" makes more sense but he originally based it on the "master copy" concept.
I personally don't understand why people decided on this hill to die on. There are much better targets like database master-slave replication.
[1] https://twitter.com/xpasky/status/1272280760280637441