What the Russian ambassador referred to when he said this was Poland's refusal to allow Soviet soldiers to pass through in the case that France and Russia came to the aid of Czechoslovakia as per anti-German aggression treaties that both nations had signed with Czechoslovakia (the Soviet's treaty was dependent on a French response). This made the Soviet attempt to organize an international response to German aggression prior to the Munich Agreement. The fact that Poland annexed a (very small) portion of Czechoslovakia that had been in dispute since the First World War is somewhat inconsequential here, except to make it look as though Poland had a devious motive in refusing the Red Army passage in the case of an international response to German aggression in 1938. But it is true that Poland refused Soviet requests to allow the Red Army to travel freely in the case of a multinational military response to German aggression and that this lessened the likelihood that the Soviet Union could organize such a response.
It is a stretch, however, to say that Poland therefore bears some of the responsibility for the Second World War. There are a few reasons why this argument fails:
It is not clear that Poland's refusal to allow the Red Army to travel across it's territory had any effect on the outcome at Munich. The French and British were suspicious of the Soviet Union's intentions, even more so after the Great Purge. Neither France nor Britain wanted war with Germany, and a potential Soviet intervention had been blunted by the purges. Indeed, the Soviets had plans to get soldiers to Czechoslovakia anyway (by air and by violating the sovereignty of other nations in the way), and this still did not convince the French or British. The Soviet Union was not present at the Munich Conference (France, Britain, Italy, and Germany were the only nations represented), and the Soviet Union remained an outsider despite, and in many ways because, the fact that it insisted on an aggressive response to German demands.
The Poles did not refuse Soviet passage so that they could work with Germany to seize a portion of Czechoslovakia. Their motives were based on Poland's historic relationship with Moscow, which had conquered portions of Poland in the past. The Poles feared that the Red Army would occupy Poland and never leave. This fear was affirmed when the Soviet Union invaded Poland and seized the eastern portions of its territory at the start of World War II.
Poland seized a small portion of Czechoslovakia that had been in dispute, but it did not launch a major invasion of another nation. Surely the nations that did so are to blame for the start of the war.