Autumn 1939: as soon as Poland has been partitioned and the new shared border with Nazi territory has been established, Stalin starts directing large numbers of his troops to abandon their well-developed permanent bases back beyond the previous border, and to move up to the new border.
This makes some sense on the face of it, because despite the non-aggression pact and especially the subsequent Friendship Pact, Stalin and every Soviet leader is very well aware that Hitler is Hitler and the Nazis are Nazis! -- and therefore as fellow post-Marxian revolutionary socialists they have the same goal of instigating and leading the workers to rise up against the property owners (rather than waiting for this to happen naturally by instinct according to Marx's original theory), supporting or providing this by armed invasion and conquest of territory so as to bring all property, materials, and means of production, eventually under the socialist control of Hitler.
One subtle difference is that Hitler doesn't seem to talk or authorize much talk, about Marx's communist stage which is supposed to follow the socialist government tyranny-for-the-people someday in the vague and distant future, unlike Soviet propaganda; but in any case, the militant socialists understand each other's threat.
Moving increasingly large numbers of troops up to the new border doesn't quite make sense of Hitler's threat, however, in some other ways:
1.) Stalin himself orchestrated removing the huge buffer zone of neutral Poland between himself and Hitler (which of course Hitler in turn agreed with, somewhat greedily and/or for short-term political convenience). Making friends with Poland and helping them gear up to defend against Hitler's aggression would make a lot more sense! True, the Poles know that they can't trust the Soviet Union, but Stalin could still have used them as a buffer zone to prepare a defense against Hitler's aggression. You could reply that Stalin knew Hitler was going to invade Poland sooner or later and so arranged to add some defensive space for Russia in Eastern Poland by taking an active role in suggesting that if Hitler is wary about going after Danzig and the Polish Corridor (so as to create a proper thoroughfare to Eastern Prussia), then the Soviets would help take down Poland, providing a deterrent to other nations getting involved at this first open armed Nazi conquest of a foreign power. However...
2.) ...that theory doesn't work because Stalin immediately throws away the extra defensive space he gained, by moving increasingly large numbers of troops, out of well-developed defensive positions, and up to the border! If they were creating new well-developed defensive positions, that would be different, but...
3.) ...building even well-developed defensive positions up next to the border is a bad idea if you don't build up supporting areas behind the front defensive line, which Stalin isn't doing; and moreover...
4.) ...Stalin isn't even building well-developed defensive positions up next to the border!
The troops are certainly very busy at construction; Suvorov cites several sources (such as the "History of the Second World War", Vol.4, p.27; "The Order-of-the-Red-Banner Byelorussian Military District [official history]", p.84, and Sandalov's "Assignment: Moscow", p.41) as examples of what they are building and why: they have no bases here, so everything has to be rebuilt and reequipped -- supply hubs and depots, airbases, the railway network (more on this and other construction soon), communication lines and centers, repair and construction of barracks and camps, new practice areas, gunnery ranges, and tank driving courses.
The MD history says the 3rd, 4th, and 10th Armies are hard at work in this area. But Stalin hasn't released funds for barracks to amount to anything, as Sandalov recalls, so the troops who can't fit in previous Polish barracks (even with new bunks stacked four-high), must make do being quartered in warehouses or even in dugouts. This is a serious problem for the coming winter!
Notice that these descriptions don't mention any defensive constructions (aside perhaps from the dugouts). That might only reflect Suvorov cherry picking his data and avoiding such references; although on the other hand, Suvorov doesn't emphasize at this point a lack of defensive construction -- he might only be emphasizing how hard Stalin is cramming troops up onto the new border area, faster than they have room to effectively be camped.
In fact the troops, under orders from high command, will be constructing defenses, even some expensive ones, on the border. More about those defenses and what else they'll be working on, later.
Meanwhile, it could be worse: what if all new divisions arriving at the border, or being created there, weren't setting up for the winter at all...!?