Yes, indeed, there is a withdrawal warning in the "Token Withdrawal After Delisting" of the announcement you referred too. And formally Bittrex can remove the wallet at any moment, and refer all annoyed customers to that warning... But practically I have noticed they try to be more cautios, and provide their customers with appropriate wallet removal email warnings - sometimes 2-3 times before actually removing wallet. Probably they try to avoid unnecessary complications, which will surely arise if they don't provide enough warnings before removing the wallet (as this could bring to a loss of funds belonging to a customer).

Not only XEL wallet is still online. I have another coin in the same list, which still has active wallet on Bittrex, despite market being delisted for few months already. As to the reasons of this behavior from Bittrex, I can think of few reasons, but I have no idea whether any of them is correct...

One thing is pretty clear - even if they didn't delist those wallets yet, they are going to do that sooner or later, unless any of the delisted projects gains REAL traction, which is unlikely.