No, we didn't (:d) réfère à "On a vu le cœur de Monet se faire planter par Ceasar). We saw the spike hit the edge of the liquidy cube. In black and white so you can't tell if its blood or cube liquid stuff. Then the camera cut away to show Monet, (who had already been coughing blood from Tashigi's hit earlier) examine the space in her heart like it had been jostled, just like Smoker and Law before her.
The very next chapter the spike is clearly impaled in the ground *beside* the cube, not *in* it.
Ceaser missed.
Oda-san framed it dramatically in such a way, and positioned the camera, to make it seem like a hit , and the anime extrapolated and made it a clear direct hit, but the manga showed no such thing. HAd Oda-san chosen *any* other angle than the one he did, a little to the left or right, or above, the hit would have been extremely clear. Instead, he chose the single possible angle where an edge grazing could look like a solid hit with some forced perspective. The man knows his reputation for not killing, so he has to try harder to make us believe nowadays.
But more important than any of that, she and Vergo did not "die" smiling, having passed on a dream, or doing anything important to the plot that would require their deaths that fainting would not cover just as well. Oda-san's tendancies just won't allow such a thing.
If she had "died" two pages earlier while smiling and completing her goal and intended sacrifice, and actually taken some enemies with her, she'd be well and truly dead. But last we saw her, there was no smile, so she's alive. ANd Vergo? Explosions have never killed, or even seriously hurt, anyone in the series, including Pell taking a city leveling nuke full brunt only leaving him with a minor chest scar. And *he* actually had a smile and an achieved goal! His death would have served a story point!
They were both out of action and seemingly killed purely so Doflaingo would write them off and they wouldn't be dragged into the Dresserossa conflcit the very next day. Them being free and clear and not arrested may be what allows Dofla free in the end.
(Also, the fact that Oda has since revealed that Monet and Sugar are sisters doubly saves her. He wouldn't sideline a dramatic sibling death and not play it up more in the character interaction. He would have had Sugar mention it in-story rather than an SBS aside.)
They're both alive and well and we'll be seeing them around chapter 800 when the next cover story starts unless Oda does something truly unprcedented like another FI story or the swirlyhats or a focus on Smoker.
This is all about how the *author* works, and the subtle storytelling tricks at work, not "Oh I love this hottie character!" The same arguments and points were made for Bon Kurei and Sabo when they were "dead" for two or three years each.