Historically, yes, this is exactly what happened. Repressive Germanization created stronger Polish nationhood among Prussian Poles where Prussian and Polish identity historically had not had difficulty co-existing before. Before 1870, I would venture to say that you would not have had to look very far to find Prussian Poles who were proud of their identity as Poles and proud of their identity as Prussians (but not Germans). Germanization made this impossible, both because Prussian identity became German-coded, to the exclusion of Polishness, but even more so, because Germanization made Poles enemies of the state, after such an event, you can't be a Prussian patriot anymore.
But, as Carlton has noted, this logic is not inescapable, because an identity of Polishness was also not an absolute, at least pre-Nazis.
Think about this as if you were an immigrant to another country. Broadly speaking, immigrants tend to follow two patterns. You assimilate into the host country's over culture, you abandon your traditional festivals for their festivals, your tongue for their tongue, and so on. Sometimes this process works better than in others; for example, America is notoriously effective for assimilating nearly everyone, in Europe, Muslim immigrants sometimes remain ghetto-ized even to the third generation. Sometimes, this is colored by race, Hispanic Americans correctly may always seem a little outsider in America even if they become monolingually English (as commonly happens after a couple generations, see Ted Cruz for a very notable example), while the sharp distinction between a Scott Gottlieb or a John Kennedy and a Henry Cabot Lodge is not so great in 2021 as it would have been in 1880. Why do you do this? Partly since as an immigrant, you're actively opting for a different country and also then a different culture than one of your birth (this makes it way easier - Poles were not immigrants to Eastern Prussia and generally did not opt to become Prussians). But mostly because in a different country, if you want to have any hope for advancement, a better life, etc, you can't really avoid adopting the dominant culture of your new country, and to some degree, often to a very big degree, abandoning your old culture. That's how very distinct German and Irish communities in 1880 America became functionally indistinguishable from broader (White) America in 2021, and how Poles in Germany could become functionally indistinguishable from Germans.
Wilhelmine Germany ITTL was not fully at one extreme - OTL they were very close to the first case, ITTL they're more in the middle.
(And very much in the minority.)