I think it's an assumption based on David Gaider showing an interest in what people think of Solas on a few occasions. For example, wondering why people thought he was named Dorian, some comments in the Twitter thread about who wrote him, his comments in the thread complaining about Solas being bald, etc. Which is a fair assumption, I guess. But the assumption might hold more weight if Gaider wasn't the lead writer, who (I am assuming - correct me if I'm wrong) oversees a lot of the writing of characters.
The thing about the "protective" theory is that, yeah, he's the lead writer, but even if he weren't, it's possible he just LIKES the character. I remember not long ago seeing Allan Schumacher saying this in the Vivienne thread, "I also have to remember that I know more about the characters than others, and while I didn't write/create any of them specifically, I still already have the ones that I consider favourites based on things that no one else knows about so the comparison is unfair." This could be basically the same thing with David Gaider and Solas.
But I'm not even saying I specifically don't think Gaider wrote Solas. He may have. It just seems like not much evidence as to whether or not he did.