They don't have to assume that traffic is efficiently routed, on the contrary if they can have a <1ms RTT from London to a server, the speed of light guarantees that that server is not in Mauritius EVEN if the traffic was efficiently routed.

It just can't be outside England, just one 0.4ms RTT as seen here is enough to be certain that the server is less then 120 km away from London (or wherever their probe was, they don't actually say, just the UK).

RTT from a known vantage point gives an absolute maximum distance, and if that maximum distance is too short then that absolutely is enough to ascertain that a server is not in the country it claims to be.