The Griever said that what they did was impossible, but Scarlet Witch just thanks to her hex magic ,which refers to a reason why Quicksilver was able to caused damage to The Griever, beacause Wanda cast out her hex to caused the improbable to happen-- that's all.
She casted out her spell as a distraction. The Griever, thinking the battle is between her and Wanda wasn't focus if another person might help.
When they were both communicating telepathically, Wanda told Pietro to run so he could hit The Griever. Wanda also challenged Quicksilver's speed, pissing him off a little.
Even Quicksilver was theorizing whether if he would survive or not by violently exploding and shattering local physics.
Wanda herself stated that Pietro shattered her essence across time. She even acknowledge his power as he is the one who can assist her in beating the Griever.
In the previous chapter, Quicksilver barely to an energy blast from the Griever.
In Scarlet Witch and Quicksilver, Vision stating Quicksilver speed harnesses physics.
Also stating that him and Wanda both grew stronger consciously or not and both have redefined their powers.
Same arc, as stated by The Wizard that every new challenge strengthens The Twins.
Same arc said that Quicksilver is evolving, learning new tricks with his speed and said that there are more to come.
Another place stating that both are without limits
Griever, Never Queen, Lorna, Vision, all stating that Wanda and Pietro strengthen each other.
The Griever as imminence said that Wanda is powerful, Pietro is the catalyst and the accelerant and when as far as calling them
Rare Beings
Something to think about is, why are they saying
Together they could affect reality but not
Wanda could affect reality?
All Quicksilver appearance from 2016 to present all point at Pietro having much potential just like his sister.