Oh, rit is definitely a gradual tempo mark—you can tell if you select it and look at the bottom status bar. It’s just that not all of the properties of the paragraph style are applicable.
It used to be that Dorico used a font style for tempo marks, instead of a paragraph style. Font styles are more limited—for example, you can’t specify a different size in the score and parts; switching to paragraph styles makes that possible. But the nature of a tempo mark object means that it follows some of its own rules, rather than the style’s rules.
If you create a text object and apply that paragraph style, it will look just like a tempo mark but will follow all of the style’s rules—but it won’t affect playback the way a tempo mark does.