Hm...some things from the article.
It appears that the fungus allows the Ork's body to negate the use of complex internal organs, the immune system and the reproductive anatomy. The fungus therefore allows the Ork to reduce the number of potentially fatal injuries possible, further improving their survivability.
This would mean even impalement would probably just inconvenience Snikrot.
The skull, like many other parts of the Ork body, is far thicker than that of a human, giving it toughness far exceeding humans' own. The tusks on the Ork face are unlikely to be related to food consumption (even though Orks photosynthesise, they also consume food, mainly Squigs) and are more for decoration or combat, more likely the latter. They have pointed ears like the Eldar, although they have a low density of nerve clusters, indicating a stripped down to bare essentials approach to the Ork creation. All of this gives Orks a simple, effective and durable body, allowing them to fight well but passing over quality of sensation.
Thus, Orks are much much MUCH more damage resistant than a human. They are also dull to sensation, meaning pain is much less...painful, for them.
An Orks internal organs, noted earlier, are simple and effective, relying heavily on their fungal elements. They have a large multi-chambered stomach which feeds a crude renal system. This combined with a body-cavity filling 'fungal soup' (which has the effect of replacing the biochemical organs (liver, kidney, spleen, pancreas etc.) gives the Orks an intensely strong internal system, even though it is quite basic in design.
As this states, an Ork is much harder to kill via striking organs, as it barely has any.
The bones of an Ork are thick, hard and durable, allowing them to sustain high velocity impacts without fracture or damage. The structure is similar to that of humans but the core of the bone contains a spongy fungal matter, replacing the simple human marrow.
So, Ork bones can resist high-velocity impacts. It stands to reason that a strike from Ajax, which would have it's force spread out more, would fail to break or fracture. Repeated strikes in one area, sure but if you account for dodges and parries from Snikrot, that's unlikely to happen before Snikrot can get a killing or crippling blow on Ajax.
Finally, much of the Ork body is filled with a spongy mass, presumably to protect what little internal organs the Ork possesses.
Further protection from all types of attacks.
The structure and design of the Ork body provide a strong, tough and resilient system but also running at a low level of complexity. These combine to show that Orks are genetically engineered for combat. Their muscles are extremely strong and their squat bodies can stand immense punishment.
Recap. Orks are tough, strong, have very few weak points, and near immune to pain. Ajax, while an incredible fighter, still has a heart and lungs and such. Snikrot is also probably strong enough to block a strike from Ajax, as long as he gets out of the way afterwords.
And there's the fact that giving Snikrot even a measly shoota would garuntee his victory, due to the sheer technology gap.