You can read Poroshenko’s statement here.
I tend to think it was a stupid gaffe/blunder on behalf of Poroshenko rather than an offense brought intentionally to hurt the Poles by exalting the UPA crimes.
Poroshenko is probably the kind of guy who doesn’t posses any special historical knowledge, it’s quite likely that he even ignored the fact that the UPA had committed atrocities against the Poles, massacring Polish civilians in Wolyn and elsewhere.
I met young Ukrainians who sincerely didn’t know the fact that Bandera and the UPA had committed crimes. We should keep in mind that the teaching of history was profoundly distorted in USSR and in all communist countries. Facts were ignored or were presented to the students in different/opposed versions, according to the political moment or the place where these facts were taught.
And if, in the time of the USSR, the UPA was presented in dark colors in the history classes in Ukraine, students might have simply assumed this was just propaganda, just one more lie, aiming at denigrating the memory of a patriotic army that had fought against the Soviets.
I remember an account by Dawid Wildstein from the Maidan. He said that, when the statue of Lenin fell down, he was close to a group of young Ukrainians waving UPA flags. When they saw how much he was rejoicing for their moment of victory, knowing he was Polish, they started shouting „Poland, Poland!”. They didn’t see any contradiction between wearing UPA colors and having sympathy for Poland.
Dawid explained in his article (Pomyłki Łysiaka, czyli co z tym majdanem?) that those young people just needed a symbol, a hero to get inspired by, some model to identify with. They thought Bandera was a brave Ukrainian, he fought against the Soviets, so did the UPA. And that was it. They didn’t hate Poland at all, on the contrary, they were thankful for Polish solidarity with their cause.
One of Murphy’s laws says that if something can be explained by human stupidity, we shouldn’t waste time looking for other explanations. I think Poroshenko’s statement is such a case.