First of all, Gwen Stefani has never released a country song. She is simply Blake Shelton's girlfriend. That has no effect on music. Pink recorded a duet with Kenny Chesney, but it was Kenny's song. Sheryl Crow has done a few duets, and she's released a country album, but it got no radio airplay. Kid Rock had one hit. One song doesn't define a genre. Blake Shelton understands that country is a fringe format. Some people know his music, but everyone knows Gwen Stefani.
But you're making musicological decisions based on people, not music. That's the same process that kept Gram Parsons from getting accepted in country music. The Byrds released Sweetheart of the Rodeo 50 years ago. It was a country record, but got no country airplay, and they got boo'd off the Ryman stage. The reason was because they had long hair. It took Waylon Jennings and Willie Nelson to break the long hair rule. Then Waylon recorded songs by The Marshall Tucker, Conway Twitty record songs by The Eagles and Pointer Sisters, and they became country hits hits. None of them had fiddles in them. If Orville Peck wants a country hit, he should sing with George Strait.