No, it's not about sola scriptura. You can use whatever source you want to prove that the apostles taught the assumption of Mary. If Catholics can't do that then they should just man up and admit it's made up from assumptions and leave it at that. That is after all what the site you sent to me says.
Here’s what you wrote:
“Please show where “the teaching” included the assumption of Mary and the requirement to believe it.”
I posted a link to that teaching. You’ll just have to live with it.