I wasn't originally going to look at it but this claim got me intrigued. I'd say compared to a lot of C code I've seen, yeah, you aren't lying, the average cleanliness is so clean it's sick ;) Well done!

Perhaps the GP means the general crypto nature of the project lends to opaqueness. For example, I randomly clicked into https://github.com/chjj/mako/blob/master/src/crypto/chacha20... and one could ask where the magic numbers on lines 41-44 come from. Maybe it's explained in the references, or just specified without explanation as part of the protocol. I looked at the reference implementation at https://cr.yp.to/streamciphers/timings/estreambench/submissi... which has the equivalent line at L66. After finding the implementation of U8TO32_LITTLE which does some bitwise-or'ing and shifting of the first 4 items ("expa" for either sigma or tau), in Lisp I quickly verified:

(format nil "0x~x"
(logior (char-code #\e) (ash (char-code #\x) 8) (ash (char-code #\p) 16) (ash (char-code #\a) 24)))
;-> "0x61707865"

So perhaps an argument could be made that your implementation could be slightly cleaner by using those sigma/tau constants instead of the magic numbers for the first four entries in the initial state, which would show how they're related, but that doesn't really take away the magic-ness of them, just moves the question elsewhere to why those magic strings. And it wouldn't surprise me if the answer is just the strings were arbitrarily chosen; the sigma/tau naming in the reference suggests a common domain convention to me (I'm not a crypto guy though so I don't know).

Still overall this is a minor point, it looks like a clean piece of code (especially given what I'm used to seeing from browsing other bits of crypto code here and there) and random sampling of other parts of the program are at least as nice and nicer, especially when they're not doing anything complicated, which I've seen plenty of C code make a mess of. While it might not always be clear why something is done, it's at least clear what's going on and where to jump for more context, what the interfaces are, and things are well-named such that I could probably go find references for some of the whys if they actually needed to be answered. e.g. in mempool.c's btc_mempool_verify, I have no clue what "Annoying process known as sigops counting" refers to, but even without that comment, the simple if condition's pieces are well-named so I could go search for more info on sigops, which I wouldn't even expect to be part of the code (at least here) anyway.

(Edit: It also occurs to me that a low ratio of comments and explanation to code could also be what is meant by obfuscation. To me that's not a large part of the cleanliness concept, though it does factor into other qualities. I consider the sqlite codebase to be some of the most beautiful C code on the planet, wonderfully documented and tested, and as a whole its beauty more than makes up for some stylistic or organizational quirks I don't exactly like. Not to say that style is totally unimportant, but neither this nor sqlite are exemplars of ugly C with lots of insane typedefs, super macros, inconsistencies, syntax abuses everywhere, and mngld_nms like vowels cost $100 a pop. For a project I consider "not so good" C code, I reference Enlightenment Foundation Libraries...)