On that topic...
I took Elidibus to be saying that simply because he is Elidibus, a Paragon of the Source, as well as Zodiark's Emissary. The Ascians, from what we know, erase/suppress their hosts' personalities altogether and it's never been implied that they felt the need to draw on their affinities to take down their foes... especially one who has already lost to you. Granted, he mentions Zenos's adamant flesh, but we know that even lesser Paragons, like Nabriales, could significantly enhance the strength of their hosts, so I struggle to see the materiality of it. If I had to hazard a guess as to what Elidibus is getting at, it might be to do with whatever lasting effects imbuing the Resonance into Zenos had on his body. It's still unclear how that might relate to it, though, since it is the soul which bears he artificial echo, but perhaps it also makes the body more susceptible to Ascian possession in some way - perhaps, better yet, such a body has been engineered so as to insulate it from Hydaelyn being able to dislodge him from it altogether. Also, he was trying to convince Varis to act faster, more than anything else. Once he realises the original Zenos is still around, he may well just give him the body back, since Zenos wants to kill the WoL anyway, but empower him with something like a mini Blessing of Darkness, so that he stands a better chance.
I also don't buy all this for another reason - essentially your mind and soul are one and the same in the setting. So you'd expect any memories and skill affinities to attach to the soul, and not the body of the host, and this is consistent with how things like the soul crystals function. Unless they were to somehow latch onto the soul of the host - really, it'd have to be closer to suppression if the body is not vacated, to avoid scenarios like Genbu's - I don't find such a storyline particularly plausible, and in fact I'd prefer they did their utmost to avoid it. I hope that if it's what going on with Matoya/Y'shtola, it is limited to her.
For the time being, with the information given, I am going to run with the assumption that Y'shtola has, for some reason, been prevented from returning to her body, and thus Matoya has taken hold of it using means we're as yet not aware of - could be the Echo, could be something else.
On...

Originally Posted by
QT_Melon
Well also don't forget
Zenos' body was taken over but Zenos appears to have spiritually hijacked a body of his own - which was one of the Elezen resistance fighters. I am still curious to that one since we haven't seen anything about it for a while
That's also why I'm starting to find the whole thing silly.
It is, a bit. As for Zenos, I'm partial to the view Elidibus will realise he's back, and eager to kill the WoL, and hand him his body back, and perhaps endow him with some manner of Blessing of Darkness to put him on an even footing with you. It makes more sense than a sudden change of heart or a throwaway fight with him, or pointlessly antagonising Elidibus and dying in the process, since for the time being, both he and Elidibus have a shared goal, plus Elidibus no doubt has his prior host, which he could move back to. The moment we first saw Elezenos, he reaffirmed his intention to hunt the WoL down. It also gives SE more room to hide who exactly Elidibus will be possessing in 5.0, if they make it sufficiently vague as to where he vanishes off to.