My Czech was terrible -- maybe just approaching B1 level when I left 3 years ago -- and it was not comprehensible to me. But it wasn't to my friend and to my (now) wife when we visited, so I took a photo and showed it to my colleagues at work and friends in the pub.
Similarly, still my favourite Czech sentence is the one that terrified me before I moved there:
Blb vlk pln žbrnd zdrhl hrd z mlh Brd skrz vrch Smrk v čtvrť srn Krč.
I memorised this (poorly) but had great fun showing it to people when I got there. Almost nobody could parse it at all.
Now, I'm an above-average English speaker. I am a very fast reader, and at school, I could cope with Chaucer just fine. Once I got my internal ear to hear it, it just made sense to me. (Beowulf, no.)
But I was the only one in my class. I can follow poetic and heavily distorted English fairly well, and it's a handy skill for a (former) professional English teacher. But I think I can follow more than most.
I certainly can't follow the ZŠ Karlstejn Czech, nor your poetic examples. I am very poor in the language. (It is my 6th foreign language, but they are all very very weak. But saying that, I've put more effort into Czech than all the rest put together, with little to show for it.)
I think you over-estimate the abilities of ordinary Czechs!