Russell is speaking of Individual Liberty and Public Control.
Thus, "thought and its expression" refers to freedom of speech.
"Statements of fact" has a similar meaning; we can relate it to scientific research :
vested interests are the principal source of anger against novelties in thought. If this were the case, intellectual progress would be much more rapid than it is.
The instinct of conventionality, horror of uncertainty, and vested interests, all militate against the acceptance of a new idea. And it is even harder to think of a new idea than to get it accepted; most people might spend a lifetime in reflection without ever making a genuinely original discovery.
Considering the fact that the book was written in 1917, we can appreciate a quasi profetic aspect of Russell's statment : totalitarian states, like e.g. Nazi Germany and Stalin's Soviet Union, made impressive and unprecedented usage of propaganda as a tool to manage power, to the point of denying "matters of fact" and produce false historical records.
See an example from Joseph Goebbels's speech to representatives of the press, 15 March 1933:
is nothing on earth that is not tendentious. Things that are not tendentious
are sexless and therefore worthless. Everything is tendentious, whether
overtly or covertly. I already believe that it is better if we admit to an overt
rather than a covert tendentiousness.
In addition, there is no such thing as absolute objectivity [emphasis added]."
See also Alternative facts.