You shouldn't feel intimidated by their density. Both J and K are extremely approachable languages, but you need to try them an open mind and act as if you didn't know a thing about good coding practices. With time you will realize that only a tiny fraction of those apply to the APL language family.
As a k programmer myself I'd say to take that statement with a grain of salt and at least give the language a chance if you have some spare time. It's certainly true that the community is small and it's certainly true that there are plenty of really sharp programmers in the community but that doesn't mean that you need to be a genius to write k code.
"Customer shall not, directly or indirectly, and shall not authorize any third party to: (i) decompile, disassemble, reverse engineer, or otherwise attempt to derive the source code of, reconstruct, or discover any hidden or underlying elements of the Software"
Looks like I don't want to spend the effort learning it.
I like oK because it allows nice toying around with graphics in the browser but while k is incredibly fast, oK is really slow. It is nice to play with and learn from but anything beyond, better check out Kona or another APL like like A+ or J.
"Shakti k also has primitives for blockchain operations."
That's... Interesting. I couldn't find any further mentions of "blockchain operations" in the reference. What do they mean, exactly? Since k is traditionally concerned with financial markets, I might guess that they're offering deeply-integrated ways to parse transactions, check the number of confirmations, query the current exchange rate, etc.?
K is interpreted, not compiled. You can run your code the instant it's written.
As other's have said, K (the language and the interpreter) is highly optimized for processing tabular data. I've heard of it outperforming C in processing billions of rows of financial data. (Which, by my understanding, is K's primary market.)
Lastly, K (like APL, J, A+, Klong, Kona, etc.) is very terse. A phrase I've seen online is "one line of K is roughly equivalent to 100 lines of C". K programs, like APL & co., very often fit entirely into a screenful of text, but require close and complete reading to understand. There are lots of videos on youtube of people writing literate sudoku solvers or conway's game of life in a dozen lines or so, and terse solutions in a dozen or so characters.
People who like these programming languages tend to like their terseness -- they can view the entire program all at once (no scrolling or searching), and refactor or rewrite the program in a few seconds of minutes of typing.
The founders of Kx (Arthur Whitney and Janet Lustgarten) sold their stakes in the company to First Derivatives plc and have now founded Shakti. Kdb+ is still sold by Kx. Shakti seems to be a successor system to kdb+.
As for you second question, how would brandelf change the binary (disclaimer: not a BSD user)?
Dyalog uses non ASCII symbols which are really cool to me and make the primitives very easy to learn. Check out tryapl.org to get a feel for it. You can also download a trial version and there is a free book. They have decent library support and full .NET support too as well as easy parsing for CSV, JSON, XML...etc. They have Dyalog APL notebooks, database access, multiplatform support, parallel and tacit features...etc. If it wasn't $1k a year for a license (honestly pretty reasonable) I'd use it pretty often. It just feels like they love their product and support it from the annual coding competitions, monthly webinars...etc.
J uses ASCII symbols and has a small community of very intelligent users that are stats smart, software smart, math smart...etc. It has great bindings to Lapack, built in graphs (well Dyalog does too) and tutorials called labs. The community is much smaller though and it doesn't have any parallel primitives. J is free to use, but the Jd columnar database is commercial, but very reasonably priced.
Honestly, both are awesome, fun, powerful, and plain cool.
Among other things, Dyalog now employs Roger Hui, one of principal creators of J, so Dyalog APL features are using full knowledge of J. Arthur Whitney's, the author of K, wrote the first implementation of J. So it's a small world...