Call me stupid, because I'm just a lowly PHP developer who's a bit confused as to what new things he wants to learn (for better career prospects), but the first reply just emphasises the balance between verbosity and terseness a language should attempt to maintain.
As a fun learning adventure it's interesting - and, as the article says a lot, mind-blowing - but I can read and understand what is going on in C.
Ask me to explain what Forth, or most if not all of the other examples are doing and you'd be lucky to get more than an, "eeeerrrrrrrrrrrrrrrr...." from me. I suppose that experimentation and innovation are more the point with this though.
You seem to think that the trouble you have reading some of this code reflects something inherent in the language (either it's too "smart" for you or simply too weird), but I would suggest the only thing it necessarily reflects is something about you — namely, that you have more experience in some areas than others.
PHP is based on C. It is not based on Forth. Thus, if you only know PHP, C code will be more immediately clear to you than Forth code.
There are some things that make languages easier or harder to read in absolute terms, but when you're brand new, similarity to what you already know pretty much dominates everything else. Similarly, English speakers tend to find Spanish easier than Russian or Chinese.
Forth examples can be more terse than C examples because Forth uses a stack for storing and manipulating intermediate values while C uses infix expressions and temporary variables. It's not just "shorter" syntax, it works in a different way. My real point with the Forth/C comparison is not that the Forth code is smaller but that it clearly has fewer "moving parts"- the for loop is simpler and provides fewer options, there's no need for special syntax to index array pointers, there's no need to specify method signatures, etc.
Language design is not simply a balance between terseness and verbosity, it's a complex series of tradeoffs between many paradigms and features- some allow you to express ideas more succinctly, flexibly or reliably.
As a fun learning adventure it's interesting - and, as the article says a lot, mind-blowing - but I can read and understand what is going on in C.
Ask me to explain what Forth, or most if not all of the other examples are doing and you'd be lucky to get more than an, "eeeerrrrrrrrrrrrrrrr...." from me. I suppose that experimentation and innovation are more the point with this though.