I don't think Rihanna is a songwriter per se, but I do think she makes changes to the demos she's given and works with her favorites (Ester Dean, The-Dream, James Fauntleroy) to develop the songs she wants and convey the ideas she wants to convey. I think the only time she was heavily-involved in writing/production was Rated R and Unapologetic. Some of the songs on those albums feel too much like "Rihanna" to solely be made by other people. But I'll gladly agree that Rihanna is usually not writing the bulk of her songs- but she's never really claimed to do so? Like her name is usually at the end of the credit list for a reason
Beyond that, why would she randomly steal writing credits on songs that the average person will never hear? What does she gain from being the second or third credited writer on a few album tracks? It doesn't make sense, especially when she has practically zero writing credits from her two biggest eras. To imply that every single Rihanna writing credit in existence is fake is just absurd