Hi there,
It flags this CSS because a significant % of that CSS is not required on that particular page.
What it cannot calculate is that some of the unused CSS is required for other pages, user login status or elements that are loaded after initial page load.
Yes you could go to the extent of creating a Critical CSS path so it loads the the necessary CSS first and defers the rest for later loading. But this is a huge task and really not worth it for such a small amount of CSS. So i would ignore it.
One thing however:
Are you using the Gutenberg editor?
If not then the Disable Gutenberg plugin by Jeff Starr has an option to remove front end styles. Haven’t tested it ( and would only do so on a backup / staging site ) but this could remove the first CSS file being loaded.