It was early 20th century, maps changed almost on a monthly basis, and many countries either lost huge chunks of territory or disappeared altogether. So why would it be strange that opportunistic countries tried to take advantage. Poland crumbled like a house of cards, they were completely embarrased and outclassed. What should have Soviets done, let their people in territories occupied by Poland fall into German hands, especially Jews. Polish government back then were proper bastards, oopressing minorities (to say the least). They bit off more territory than they can chew and it backfired spectacularly. The tweet is correct.