Walkinshaw has just said that the two men who were with Markenmore were strangers. Chilford's response is:

You cannot be sure they were strangers. Markenmore might have known them from before, as he has business dealings with a wide range of people. You are assuming that Markenmore came here to meet his brother and sister. But it is equally probable that he came here to meet the two men.

Hope that clarifies the meaning a bit!

Walkinshaw has just said that the two men who were with Markenmore were strangers. Chilford's response is:

You cannot be sure they were strangers. Markenmore might have known them from before, as he has business dealings with a wide range of people. You are assuming that Markenmore came to this area to meet his brother and sister. But it is equally probable that he came here to meet the two men.

I.e.: the usual assumption is that Markenmore came to visit his siblings and stopped at the Sceptre Inn as an afterthought. But perhaps it's the other way round: maybe the main reason that Markenmore was in the vicinity of the Sceptre Inn in the first place was for a meeting with the two men, and he visited his siblings just as a blind, or because by chance the chosen meeting place was near them.

Hope that clarifies the meaning a bit!