I read some posts on a German martial arts forum about some grapplers (MMA and HEMA)
So, in other words, not necessarily highly skilled grapplers?
who tried to play around with Pankration (ie. only eye gouges and biting are banned). Meaning: They tried to recreate the style unprofessionally by looking at statues, vases and old books and using their own expertise and skills to fill in the gaps.
Most certainly not high skilled grapplers then.
For example, the guard was easily countered by punching the bottom guy in the balls several times.
Limb control is a primary principal of good grappling. If your opponent is in your guard and has time to sit up and start throwing punches then, again, you're failing in your control attempts. You need to control the wrists and the head as well as using your legs to off balance your opponent. I'm having a hard time (heheheheh) effectively visualizing such strikes from a closed guard scenario, so I'm assuming open guard.
they expected Pankration to be a kind of watered-down MMA, but it turned out that Pankration was a much more brutal sport where anyone using a modern approach was basically completely lost. It was completely incompatible with modern ideas of combat sports. Moral of the story: You have to change your mind set completely when there are no rules.
You're ignoring the possibility that these individuals, whose knowledge was limited to analyzing vases, were insufficiently skilled to demonstrate the conclusions you're making. If I took two white belts, told them they could hit each other in the balls, and set them loose, I would expect a similar result to what you're encountering. However, that's because they aren't good at grappling. To analogize it differently, beginners in the dog brothers stickfighting classes that knew some grappling would bum rush, lock sticks, and then turn it into a BJJ match. This doesn't mean that this was the alpha strategy, but rather that they sucked at stick fighting. When they went against a real stick fighter, they couldn't get close enough to do it without taking some cracks in the dome.
Pankration featured back attacks, leg locks, and other grappling maneuvers, in this self-same referenced vases/historical documents. Are we to assume that this was in a moment immediately preceding a devastating, game ending groin strike? No, that's silly.
When one does a google image search of "Pankration", it is surprising that not a single historical image features groin shots, even ones where people are held in a guillotine like position, ripe for the groining. Or that someone is depicted finishing a single leg, instead of reaching for the groin. Indeed, one would ask, if the groin grab is the end-all-be-all to counter grappling, pankration should have taken a stylistic approach such that the game revolved around it; the ball grabbing meta-game would change the posture, stance, and submissions of the sport. Both men would stand bladed, with hands at a low level, ready to reach for the groin. Grappling exists only in so far as you are forcing your opponents legs open to expose his delicate loins. Submissions are wastes of time and effort, as groin grabs are both the counter to submissions, AND the only submission worth pursuing.
But that didn't happen, did it? It would seem to imply, instead, that they have less of an impact than you would imagine. Especially against the hard men who fought in pankration.