Your source is some guy on the internet?I actually found the evidence from the following forum. The thread host listed a number of Polish books as bibliography. (though I am unable to check the claim since I don't know Polish and certainly could not find any Polish book in the region I live) Let me show his article here anyway.
Source : https://forum.axishistory.com/viewtopic.php?t=1960
In the wake of the Versailler Vertrag, many Germans found themselves living in Polish territory and ruled by Poles. Poland was highly hostile to Germany and especially the German minority which found itself dislocated, something that spawned from even before W.W.I but would reach its peak after the first World War.
The first Polish atrocities against Germans took place during the what is called “dritten polnischen Aufstands” (the ‘third Polish uprise’) in Upper Silesia in May and June 1921. (1)
On the fifteenth of May 1927 an anti-German pogrom took place in Rybnik. (2)
Starting from April/May 1939 regularly assaults started taking place, the atrocities were no longer sporadic but the increasing hate-feelings of the Poles started to show. Several months before Germany invaded Poland, the news and radio services in Poland spread the message that; “daß im Kriegsfalle kein einheimischer Feind lebend entrinnen wird”. (3)
“In the case of war, no ethnical enemy (meaning the Germans living in Poland) will escape alive.”
Also before the outbreak of war, the Poles constructed two concentration camps where the German population was to be brought too after being arrested, and, if we listen to the Polish media, annihilated. One was situated at Polowanie, the other one at Niemcow. (4)
The outbreak of the war on 1.9.39 between Germany and Poland was to seal the fate for a lot of Germans. The hunt against them began immediately, as planned by the Polish authorities. The main centre of outbreak of these pogroms was the city of Bromberg, where German inhabitans were slaughtered like beasts. This day is known in German history as “Bromberger Blutsonntag” (5). Lodz, the Polish corridor and Ostpreußen were also the background of Polish deportations and atrocities.
Deportations started with lists of all German residents who were to be arrested and deported. These lists had been long prepared. Officially, the ground on which these people were arrested and deported concluded ‘espionage’ or ‘subversive activity’. The arrested civilians were brought to Eastern-Poland on foot-marches. Those, who could not follow, were struck dead. Of the 700 arrested Germans from Obornik, 231 were killed during the march (6). The perpetrators of these acts were Polish policemen and paramilitary youth-units.
The Ukrainian minority in Poland also suffered from these attacks by Poles. (7)
Poland now admits that these atrocities took place, and the government has come up with the number of 3.841 casualties. German sources, however, established a total of 5.490 deaths and missing people, with hints that the total number is likely to be over 6.000. (8)
1. “Die Geschichte der polnischen Nation 1918-1978”, Hans Roos, p. 180.
2. Alfred Bohmann, “Menschen und Grenzen”, p. 38.
3. Peter Aurich, “Der deutsch-polnische September 1939”, p. 48, Theodor Bierschenk, “Die deutsche Volksgruppe in Polen 1934-1939” p. 319
4. Zayas, Alfred M. de/Rabus, Walter: “Die Wehrmacht-Untersuchungsstelle” p. 249
5. Mühlfenzl, Rudolf, “Geflohen und vertrieben”, p. 36
6. Nawratil, Heinz, “Schwarzbuch der Vertreibung 1945-1948”, p. 43-52
7. Ibid. op. cit.: de Zayas
8. Zayas/Rabus, p. 244 (an estimated 4000-5000 casualties) op.cit.: Wehrmacht-Untersuchungsstelle + Schubert, Günter: ‘Das Unternehmen "Bromberger Blutsonntag" ‘, p. 199.
Obviously, violence, racism and discrimination were not the patent of Nazi Germany. The only difference was the latter make them much horrifying with magnitude in scale.