It’s only diminishing if it’s qualifying rather than describing or even amplifying.
Paula Vogel and Sarah Ruhl are female playwrights in the same way that Tony Kushner and Lin-Manuel Miranda are male playwrights. Being female playwrights doesn’t make them not playwrights (although it does make them statistically less likely to be produced). August Wilson is one of America’s greatest playwrights, and he is a Black male playwright. Those two realities aren’t mutually exclusive.
To ask some more questions: What is the problem with being identified as a Black playwright, except to people who view Blackness as less than? Why would we ask ourselves if we “must always” identify August Wilson as Black, as if that is shameful rather than beautiful, or at least factual and respectful? August Wilson would insist that we acknowledge his Blackness, rather than pretend he stopped being Black when he became a genius - and he would (and did) ask us to shift our funding models to support a true Black Theater in America that rivaled Broadway.
Acknowledging gender and race is only a qualifier if we view “Playwright” as the domain of white men, or if we view being male and white as being neither a gender nor a race. This is a way of thinking that many of us have inherited from the system we live in, and we have to face that in order to move past it. The goal is not for women and BIPOC artists to transcend gender and race, the goal is for the rest of us to acknowledge that we have to face gender and race in our own existence.