Even before the threats made to Upwork, I feel the 'client' was trying to trick the freelancer.

- Snag a freelancer without providing spec up front. - Once some desperate freelancer signs on, flood the freelancer with tons of work. - If the freelancer tries to back out, threaten that you will file complaint with Upwork. Since freelancer was desperate enough to sign on, the client probably assumes the freelancer will be desperate enough to suck it and finish the work. - Repeat. Hence 40 previous jobs.

I think some call this 'client' a shrewd businessman.