The TCP RTT data comes from the Host sFlow agent,
https://sflow.net. The tcp{} module provides the data using Linux tcp_diag instrumentation.
With Mininet, the embedded agent in Open vSwitch provides the sFlow measurements and it doesn't support the measurements since they can only be made at the TCP end points. Mininet doesn't fully containerize hosts, so I don't think it would be possible to run Host sFlow agents with Mininet host nodes.