By that measure, APL programs must be the least buggy in existence.
I'm not sure if K generalizes well. It may be that plenty of software has edge cases that must be handled that don't have any elegant representations. I have a hard time believing such little code can actually work that well. But it seems impressive enough that I plan on learning it some and making sure I'm not massively missing out on something because it seems foreign.
From my own experience, less code certainly means less bugs. Even with denser code. Simply having to jump around to find stuff (due to more lines) seems to cause more problems
From this perspective, just factoring out imperative code into a named subroutine becomes a pain point because it adds new symbolic content - a new name, things being passed in, things being returned, things that were in scope suddenly falling outside of it. It is no longer an "organization tool" - you can add a level of block scope and a comment for that instead, and navigate the resulting large functions with code folding. And classes - well, classes become something to avoid!
APL style simply moves this concept down to the per-character level of thought and advocates additional care towards certain measures of readability. "It is more likely," says the APL programmer, "that you will see a typo in a 2 character name than in a 10 character name." The language aids in this process by providing tools that drastically condense common operations and encouraging their use as the default.
I just hit a major bug due to that kind of thing, where stuff wasn't clear due to messy size of the code (customer insisted on C#). If the relevant sections had been sitting on screen, then I most likely would have noticed or never even made it in first place.
I also started working with a new hire, one that isn't an FP proponent, and I'm shocked at the variable names. Even iterating a list of entry objects: I'd naturally write the list as 'es', and the current item 'e'. Versus 'entriesToProcess' and 'current entry'. Or even 'entries' and 'ent'.
People's objections there seem to be that 'entries' is more readable than 'es'. While that may be true in isolation, I'm not sure it matters when working on a codebase above a few hundred lines. The amount if context is likely to be high enough that you're not going to be plucking one line out for changing, and the local context (and typing) should make it apparent it's a plural of e.g. entries, not employees.
C# the language is superior in essentially every way to Java. Far less code = less bugs. F# takes it up another notch, while allowing you to keep interop.
Microsoft has had plenty of opportunity to officially support Mono, but they have not. Their patent promise only extends to Mono as long as you don't run into other issues with Microsoft. If MS wanted, they could, at any time, start really supporting Mono and work to unify runtimes. But anything's possible.