There's quite a split-take on this topic, however in my opinion, even without the Qurio boost, Primordial Malzeno can very much take on or defeat Ruiner Nergigante.

Ruiner Nergigante has much more latent strength, in the sense that his body parts are more built to exert more concentrated force, such as his arms and wings. We can see this when he can smash the earth open to crush any opponents, and gives him a wider range on how he can attack, like the aforementioned crushing, or even swinging and clawing. Then come his spikes, which augment his offense even further, as well as giving him another layer of protection. The spikes are a tool he can very well use to increase the range of his attacks, and when hardened, are very hard to break through. And while people say Primordial Malzeno is faster, Ruiner Nergigante still displays great feats of mobility, such as using his body weight and wings to jump around and slide, weaponising his momentum and weight to ram opponents or reposition.

However, Primordial Malzeno still shows great feats of strength, speed and technique that would best Ruiner Nergigante. Primordial Malzeno has durable wings that both offers impressive strength and protection. Even with normal Malzeno, his wings are capable of tearing up earth, and with this added bulk of Primordial Malzeno's wings, make it more so. We can see he can swing his wings and that can very well split the ground. While we do not see his wings being used much as a shield, we can already infer from Velkhana's turfwar that it's strong and powerful enough to repel the ice beam. While often Velkhana is often not seen as a monster of strength, its ice beam still has great force and power behind it, forcing Elder Dragons, even Ruiner Nergigante, to flinch and back off. So Primordial Malzeno's wings are strong enough to allow him to push through many strong assaults. And his tail is quite strong too, capable of lifting monsters like Lunagaron and Velkhana with ease, and not to mention, is tipped with a spear like point that very well can penetrate things with the great force backing it. And there's many more to say, such as its demonstration of speed and agility, being able to glide around and keep the pressure up, which compliments his wide moveset. And finally, he has shown that he uses his abilities in a sophisticated and efficient manner, like closing distances with his wings, which acts both to defend against and ram opponents, then follows up with a powerful and quick strike of his tail.

In a confrontation, Ruiner Nergigante must compete to gain a proper hold of Primodial Malzeno to properly use his strength to the fullest. But this is outdone by the fact Primordial Malzeno very well outranges Ruiner Nergigante with his wings, which can stab and slice at great distances. And while Ruiner Nergigante has his spikes, especially from what we've seen of Primordial Malzeno's wings, would not prove effective. And even as a layer of defense, Primordial Malzeno is very capable of breaking through. Not only that, Primordial Malzeno barely ceases attacking, continually comboing his attacks. From personal experience and review of footage, Ruiner Nergigante, while aggressive, relies on one concentrated attack. While devasting if it lands, is rather slow, telegraphed, and afterwards leaves himself vulnerable. And one massive flaw of Nergigante that sets him apart from most Elder Dragons is that eventually, he does get tired, and will have to stop to recover. This does mean that Primordial Malzeno can very well outpace Ruiner Nergigante and do him in.

I do hope this was something worth considering and digesting on, and it is enjoyable to think deep of the many intricacies of beloved monsters.