The global problem is not the stuff that is in a good enough condition for resale. It's the stuff that isn't and still ends up in the donation bins.
I'm arguing that what the author is saying is largely false.
Your counterpoint to the authors claim that nobody wants your used clothes anymore seems to be that a handful do, which is great but is a drop in the ocean compared to the amount of new product being churned out, consumed, and ultimately dumped - albeit in some cases via a donation bin or even a cycle of reuse - via the fast fashion chains.