Of course readability matters. But you must know language constructs anyway, no need for them to be overly explicit and be in the way of your code - therefor it could easily be argued that "mtx" is easier and more optimized for reading than "mutex".
The only reason an "if" or "while" statement makes sense to every programmer isn't because it is readable but because you must know the syntax and semantics of the language you program. And having a good shorthand is faster to read and since mtx isn't a word it isn't associated as anything that could be a word but you instantly see it as a language construct (kind of like syntax highlighting) and thus reduces the work of mentally parsing the code (one advantage of not writing your code in English is that you subconsciously see the language constructs differently than any of the code that you wrote yourself, not saying that I prefer to do this or recommend it (on the contrary)).
Again, this example alone isn't one I'd really care about - just questioning that "mutex" would be more readable than "mtx" (as a language construct).
I think I'm going to flip this around and say can you argue for why the keyword shouldn't be to use the "default position" of using the real word when its only two characters more?
The only reason an "if" or "while" statement makes sense to every programmer isn't because it is readable but because you must know the syntax and semantics of the language you program. And having a good shorthand is faster to read and since mtx isn't a word it isn't associated as anything that could be a word but you instantly see it as a language construct (kind of like syntax highlighting) and thus reduces the work of mentally parsing the code (one advantage of not writing your code in English is that you subconsciously see the language constructs differently than any of the code that you wrote yourself, not saying that I prefer to do this or recommend it (on the contrary)).
Again, this example alone isn't one I'd really care about - just questioning that "mutex" would be more readable than "mtx" (as a language construct).