As far as I can tell the main reasons for APL being written like it is are 1. it's ancient, from the world of teletype where character count was way more critical, and 2. some programmers love code-golf write-only syntax.

It reminds me a lot of regex. You could say "there are good reasons regex is so terse" and "people successfully use regex all the time" but that doesn't change the fact that it is a very write-only syntax and would be much better if it was more verbose. There are actually a lot of recent efforts to do that.

The jsource repo you linked seems to have had only 4 contributors ever, which suggests to me that it is not a popular style and not easy to read.

As far as I can tell APL had some interesting ideas in terms of data manipulation, but there's no reason those ideas have to be expressed all on one line with no comments or spaces.