It doesn't make sure the nodes along the route do not drop the packets. Nodes along the route would see those packets as normal TCP packets (which is by design

It doesn't make sure the nodes along the route do not drop the packets. Nodes along the route would see those packets as normal TCP packets (which is by design because in this way routers in between don't have to change). But both end are aware of MPTCP and they may use multiple path. In this case, iOS is connecting to Siri server using both 4G and WiFi. They apparently go through different routers. This helps improve reliability when the device is at border of a WiFi coverage and is switching between 4G and WiFi back-and-forth. Using two path simultaneously would allow Siri to switch to the other route when one of them is showing significant packet loss.