The buyer can not mark an item as delivered. If you haven't left feedback yet it 'might' help for the seller's money to be released sooner. Other than that, there is not much that you can do.
If the seller contacts customer service, they may look at the message you sent to the seller confirming that you have received and are happy with the item.
That thread you referred to is from 2019. Old threads often have outdated information which is one of the reasons why they get closed.