The methodology is "(likes + shares + comments)/followers", and the two platforms that allegedly provide the highest engagement are the ones with an order of magnitude fewer followers.
A more likely explanation is that fewer followers means higher engagement, which totally makes sense; for a larger social network account with 100k+ followers, many of those will be bots and people who follow anyone and everyone without really caring. A smaller account will only have followers who care.
I don't think there's anything specific about Mastodon here other than it being a smaller social network so you naturally have far fewer followers.
In either case, this is a single account talking about a single post, and shouldn't be used to generalize different levels of engagement across social networks.
A more likely explanation is that fewer followers means higher engagement, which totally makes sense; for a larger social network account with 100k+ followers, many of those will be bots and people who follow anyone and everyone without really caring. A smaller account will only have followers who care.
I don't think there's anything specific about Mastodon here other than it being a smaller social network so you naturally have far fewer followers.
In either case, this is a single account talking about a single post, and shouldn't be used to generalize different levels of engagement across social networks.