Part of those two days of commenting was that since K is interpreted all of the developers would hand-obfuscate all of the code so that the variables were a, b, c, etc. so that they would be slightly faster to parse than a multi-character string. This sort of thing is done all the time now, with JS to make it load faster, but his team at least didn't have any scripts to automatically turn developer-friendly code into interpreter-friendly code, they worked with the interpreter-friendly code version.
The notation-as-a-tool-of-thought camp prefers short identifiers on the principle that the more concise the expression, the easier it is to comprehend in total. This is not at all dissimilar from the general practices of mathematics.
mine was matlab. thinking of everything as an array, especially things that aren't arrays, was the first pattern I ever learned. I've never used APL in anger but for the most part it comes very easily to me. Maybe not the code golfing and/or idiomatic APL style that people like to post but, like, using the primitives to create increasingly elaborate arrays before discarding anything I don't want, that part has always felt very straight forward.