Meaning the original meaning of the word: not credible. Instead, nothing more than a stunning example of projection. I can only wonder what the producers see as they watch clouds roll by.
As for the Masma and their civilization, if they couldn't do better than the examples shown in this program, they don't rate much notice.
Actually, their existence is as problematic as their monuments (and, actually, I'm just filling out my ten lines).
One late discovery does show how sophisticated hunter/gatherers could be: Gobeki Tepe. The monumental sculptures discovered at this tell in the 90s are unmistakable, extremely impressive and show what can be done using nothing more than stone tools. Unlike the stones at Markawasi, there is no need to squint into the sun before you can begin to see even vague figures.
Again, this show is about the strength of projection: only one of the examples shown has the vaguest similarity to a human profile; all of the others are nothing more than piles of stones resembling nothing but piles of stone.
The producers do acknowledge the projection but call its effect a natural occurrence, which they call simulacra and genius loci, in which 'Man's' propensity to imagine figures in nature is mirrored by 'Nature's' desire to provide formations for 'Man' to anthropomorphize.
And then they project the image of a horse on yet another formless formation just to show that the Masma sculpted an animal they had no knowledge of, especially since the Masma pre-date the domestication of any animals, except, perhaps, dogs. Of which there are no pictures at Markawasi.
At any rate, there are truly astounding finds like Gobeki Tepe and Tell Brak that enrich our knowledge of genuinely ancient cultures without the need to project Dumbo on every passing cloud.