I'd need to do more tests or see a breakdown to say for sure, but I imagine Flarum's results will be impacted a lot by the database server since the homepage requires a number of SQL queries to be performed. Those queries would obviously take the same time to complete, no matter the PHP version.
I also wonder whether they visited Flarum before the initial query. Because if the CSS and javascript files aren't compiled, the first query will take a huge hit. If they just installed Flarum and didn't enable/disable any extension then it should be fine because the first redirect after installation should take care of generating those files.
I see in the article they say the Laravel test used few or none SQL query though, so not sure why it seems to correlate.
I'm also not sure why they marked 8.1 as unsupported. Officially it's supported, PHP display_errors
just needs to be turned off.
EDIT: I see their screenshot is labelled "The tested Flarum page" and we see a user is connected. So this means they tested the logged in experience instead of guest (?) Also if they tested with an admin user the performance will be worse and even more database-tied than a regular user because more data is retrieved.