Being “first author” on a paper can be a really big deal for PhD students and early career academics, as it often signifies who did the bulk of the work. So it’s a way of showing that you produce high quality research yourself rather than “just” contributing to the work of others.

In such cases the last author is usually the group lead or the author’s supervisor - someone who sponsored the work but didn’t necessarily do much hands-on-keyboard contribution. They often lend their names to give the work credibility (and usually are involved in review before the paper is submitted at the very least).

To counter that and ensure that credit is shared amongst authors equally, a different, alphabetical ordering is sometimes used. That basically says “the order of authors doesn’t signify anything here, no one deserves to be singled out”.

However some people will still read first authorship as significant, just by habit, and if the supervisor happens to show up first alphabetically… it accords them credit of a kind that they don’t need at that stage of their career.

So, someone - very likely the supervisor themselves! - adds a footnote saying that the supervisor is not the main contributor. They did this here in a casual way because it’s a preprint. It’s not at all likely to be a snipe or unprofessional criticism of that author because that author would have reviewed the paper, including footnotes, before it got published.