When he became a venezuelan citizen in 1951 he had supposed lost his polish citizenship, but, because he didn't joint any other army in any country, and because he didn't get authorization from the polish military offices to became a citizen of Venezuela, he was still polish to the eyes of the law.
That's the difference between your case and his. Poland did not exist in 1900, at the time he left Europe for America he travelled as a citizen of Austria-Hungary born on the territory of Poland because that part of Poland was legally under their jurisdiction and administration. His great grandfather never resided in independent state of Poland having left before such existed therefore never been able to apply for nor having been granted automatic Polish citizenship by residing in that country. He has no way to prove he ever was a citizen of Poland, the travel documents in his Possession were issued by government that is other then Polish. In your case whether or not he possessed Polish citizenship at one time or another was never at question only if he legally lost it or not, technicality quite different.