MultipathTCP: Am I doing it wrong?

If I set sessionConfiguration.multipathServiceType = .handover trying to access http://amiusingmptcp.de (or, for that matter, any other site), I am getting the following errors:

MultipathTCP nw_socket_set_common_sockopts setsockopt MPTCP_SERVICE_TYPE failed: [13] Permission denied
MultipathTCP nw_socket_add_input_handler Failed to initialize socket
MultipathTCP nw_endpoint_flow_attach_protocols [1.1 130.83.166.249:80 in_progress multipath-socket-flow (satisfied)] Failed to attach socket protocol

If I set sessionConfiguration.multipathServiceType = .interactive, the errors are similar, but appear twice.

MultipathTCP nw_socket_set_common_sockopts setsockopt MPTCP_SERVICE_TYPE failed: [13] Permission denied
MultipathTCP nw_socket_add_input_handler Failed to initialize socket
MultipathTCP nw_endpoint_flow_attach_protocols [1.1.1 130.83.166.249:80 in_progress multipath-socket-flow (satisfied)] Failed to attach socket protocol
MultipathTCP nw_socket_set_common_sockopts setsockopt MPTCP_SERVICE_TYPE failed: [13] Permission denied
MultipathTCP nw_socket_add_input_handler Failed to initialize socket
MultipathTCP nw_endpoint_flow_attach_protocols [1.2.1 130.83.166.249:80 in_progress multipath-socket-flow (satisfied)] Failed to attach socket protocol

The session will load data just fine, if I set multipathServiceType = .none

Are there other things I need to do in order to use MultipathTCP?

Using Xcode 9.0 beta 4 (9M189t)

EDIT: I did enabled Multipath Networking in Settings->Developer, and rebooted the device.

This is still happening with iOS 11 Beta 5 (15A5341f)