Some of Mastodon's upstream decisions strike me as "we must be identical to Twitter in the insignificant ways if we want to get users".
At one point, Gargron, the main guy behind Mastodon, even mentioned using a trumpet as the basis of Mastodon's logo branding: https://github.com/mastodon/mastodon/issues/962
"Toot" also has the double entendre. In all my life, I've never heard anyone outside a playground use it to describe flatulence. In my personal experience, it's almost always been about the musical notes. I'm not in a band or otherwise around musicians more than the average person, either.
There's nothing more to it than that: A quirky name based on the chosen logo, following the Twitter example.
I agree it wasn't the best word choice, and I can understand why it has been de-emphasised in favour of 'posts', but the reasoning behind it was logical.
Hard to resist, isn't it.
Similarly Bluesky did its best to get rid of "skeet" before blowing up haha