They are restored but not used, only when Tippi teleports then to Mario it works, they don't stop Super Dimentio weakeness inside of it.
The four heroes were reunited in the scene where Tippi and Bleck were married.
It is a weakeness because Mario would have broken Bleck's barrier without the Heroes being reunited, everytime they were used, which is diferent from being restored, the four heroes were there, it's stated pretty clearly that when the four heroes are reunited shall they defeat the Chaos Heart, if they didn't need to be togheter they would't need to find the heroes and Mario would have broken Bleck's barrier, Tippi only restored the Pure Hearts in Dimension D but didn't actualy use them, only when brought to Mario and Co. they worked against Super Dimentio, and in the wedding scene the four heroes were present, they being powered by love isn't a contradiction.
Again, if they could use them by themself Mario would have broken the barrier, which didn't happen, only when the Heroes were reunited is when they worked, not when Mario was alone
So to resume, they being powered by love doesn't contradict the fact that Merlee states that only when the four heroes are reunited shall they defeat the Chaos Heart, the fact that they didn't work when Mario was fighting alone against Bleck, only when the Heroes came back, and Dimentio being weakened only when Tippi brought them back to Mario, where the four heroes were( though Luigi was Super Dimentio it doesn't contradict the prophesy, in a way all four heroes were there) and during the wedding scene the four heroes were all there
The only reason I am saying all of this is because the Pure Hearts only worked because all the four heroes were reunited, when Mario fought Bleck by himself they didn't work, Mario couldn't even scratch his cap as stated by Bleck, only when the rest came was when they had the power to beat Bleck, so the only time when the four heroes weren't in the same room they didn't work, and all other times they were in the same room and the Pure Hearts worked, the scene in Dimension D was Tippi restoring the Pure Hearts to life after they already were used, which brought them to a grey state of powerless, restoring something from powereless, and using it it's diferentely, otherwise Tippi is 2-B, which she isn't