The quote appears in a 2001 paper in English... and only there in that exact formulation.
[Stalin's] policies during and after World War II amounted to “National Bolshevism”—the use of the Soviet state as an agent of revolutionary change. Stalin himself said as much in 1940: “The action of the Red Army is also a matter of world revolution.”
It's footnoted to "Dimitrov, Dnevnik, 188", which I think refers to the memoirs of Georgi Dimitrov, who at that point (1940) was the General Secretary of the Communist International. The author of this paper cites a good number of sources in Russian, so I think he reads the language, and thus may have included his own translations. Therefore, the fact that the (bold) quote doesn't appear as such anywhere else on the internet doesn't necessarily imply it's inauthentic; possibly there are other variations/translations of the original, but on a quick search I was unable to find any.
The Diary of Georgi Dimitrov was (interestingly) published in English only after that paper (namely in 2003). However, the page numbers do not remotely correspond: p. 188 is in the middle of 1941 in the English translation.
So, does the memoir have that quote (in some variation)? Did Stalin ultimately say/write that?