I don’t think he had anything to gain by faking it. I’d like to believe it was sincere.
I am a lifelong Polish-Catholic. Kneeling is required at certain parts of the Mass, regardless of whom the Mass is offered for. It’s required and it’s universal.
And the question is, if hardliners had stayed in power in the Soviet Union, instead of Gorbachev, would Jaruzelski have softened as he did?
But, having said that, yes, perhaps with the years passing, he truly saw the error of his ways. Now one might say, why did he wait until he knew he was at death's door to return to the faith, but I'll leave that between God and him.
And if indeed he goes to Heaven, may Father Popiełuszko be the first to welcome him.