I'm not aware of a backdoor HSA withdrawal method.
I do know that you can withdraw from your HSA for any medical expense in any year. So lets say you are doing well this year, and you end up with $5000 in medical bills for 2020, you can save those receipts and in 2025 withdraw the $5000 if you wanted to spend it on something else, since you would be simply reimbursing yourself. As long as you had the plan active during the time the expense occurred you could reimburse yourself at any time.
But honestly, how on earth can they keep track with hundreds of millions of people having HSAs in america making small withdrawals?