Earlier quoted context omitted.
That's perfectly readable to me as a speaker of modern Czech, and it appears to have been readable to all the commenters on your post too. > W téchto se dozwédá sjnjch pilné robé, Bohu čjm povinné, bratřjm, wlasti, sobě. This is almost modern Czech, with j for í and w for v (which was the standard orthography for hundreds of years and would be familiar to any educated Czech). Take the most famous poem in Czech, in it…
It's fascinating, and thank you. 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…
> Blb vlk pln žbrnd zdrhl hrd z mlh Brd skrz vrch Smrk v čtvrť srn Krč.
I do enjoy this one. It's all the more amusing in that it only uses fairly common vocab (though somewhat stylised inflection).
Speaking of parsing, you may be aware of the perfectly valid English sentence "Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo". Or the sentence "The horse raced past the barn fell". If not, enjoy. :)
> I think you over-estimate the abilities of ordinary Czechs!
Very possibly! I think two of the examples I provided (the poem Máj and the quote from the Kralická bible) would genuinely be comprehensible to most Czechs, but I might be being too generous with the Karlštejn quote.
If I render it into standard orthography, give or take some endings I had to guess by squinting at the image on my phone, it comes out as:
> V těchto se dozvídá síních pilné robě, Bohu čím povinné, bratřím, vlasti, sobě.
Reordered to simplify the structure, and with glosses, you can probably make it out:
> V těchto [in these] síních [halls] se pilné [diligent] robě [child; archaic] dozvídá [learns, finds out], čím [that which is] povinné [compulsory] Bohu [dative, i.e. to/for God], bratřím [to/for brothers], vlasti [to/for the homeland], sobĕ [to/for oneself].
There is some basic vocab here (Bůh, bratr, vlast), some intermediate vocab (dozvědět se, pilné), and one clear-cut archaism (robě). The overall structure is quite poetic and somewhat inorganic - e.g. the separation of těchto and síních in "V těchto se dozvídá síních" is unnatural, and makes me think of scholastic Latin.
I'm honestly not sure how an ordinary person might read this. I think if it were in standard orthography, it would probably be fairly readable, but if you layer pre-modern orthography on top of the structural acrobatics and archaisms, you're probably right that the average person may struggle.
Čapek uses pre-modern orthography (e.g. gegj for její) for stylistic effect ('ye olde... ') in small sections of Válka s mloky, and modern editions do not generally transcribe these sections into modern orthography, so I think there is at least the implication that a literate Czech ought to be able to read it. To what extent that expectation has a relationship to reality I'm honestly unsure. The Czechs can be (far too) normative when it comes language, as you probably know, so perhaps the literati insist it be printed as is, and everyone else is just too scared to admit they can no longer read it. :P