Most programmers find k interesting. k is fast. k is small. k & APL are great as a tool of thought. It has a "lost art" feel. It's an intellectually satisfying endeavor.
Do many people here deal with array computation?
Like three Dyalog employees occasionally contribute to discussion, fintech employees make up like 10% of the people regularly writing comments on the site, and array programming is incredibly popular when in disguise and made less efficient as Python (numpy), pretty much essential for multiple subfields of computer science, so forth.
K is also supposedly fast—not once have I seen an explanation of what makes it fast
People talk about it all the time here, though. 'geocar mentions why in almost every thread, and he should know: he helped write an operating system in the language.
and why those techniques aren't looted for open-source languages.
Because they don't want to believe it to be true! When told you're messing up something simple, you have to question your assumptions a bit. People hate doing this, especially when they've invested time in seeing them through.
Is K in fact a thin layer on top of assembler (specifically SIMD and other extensions), for number crunching in the vein of shaders/CUDA?
k is higher-level than C, Haskell, Rust, etc.