Most likely because of magic

I'm curious. Where in canon does it say that 丹枫 managed to fight 镜流 on equal grounds? Not fan speculation, but actual canon statement. Like where's the source for this?

And these are fans, not writers, so they're not required to know good writing.

If fans want to have conversations about the quality of the canon writing, then, akshually, yes, yes they are required to know good writing.