I recognize the original post itself claims that Ral is a tomboy here, but should we consider her a tomboy via Danbooru tag-what-you-see standards?

There's little to distinguish any of these poses or uniforms from any of the other non-tomboy girls playing_sports to make me think this post should be tagged tomboy

Tomboy is one of those tags that seems to get overused an awful lot. Artists will call a character a tomboy without actually drawing her as one, and uploaders frequently just roll with it for whatever reason. If you see a girl tagged tomboy who doesn't look very boyish and isn't behaving in a typically masculine manner, then by all means remove the tag.