Well back then Lviv (Lwów) was a Polish city. In Yalta, that part of Poland
was taken from Poland (and "in return", borders were moved to the west). Many
a person was forced to move from Lwów back then...

Personally I find your supposition amusing -- Lem thought about himself as
Polish citizen, spoke Polish, wrote in Polish, lived in Poland. Both died
and was born in Poland.

This reminds me of a joke, which went something like this:
  -- Tell me, are there any famous Polish people?
  -- For instance, Nicolaus Copernicus.
  -- He was German!
  -- No, no, he was Polish! Another one: Fryderyk Chopin, the famous musician.
  -- He was French!
  -- No, no, he was Polish! Say, another one: pope John Paul II!
  -- He was Italian!!
  -- No, no, he was Polish! ... and so on.