Yes, it's not C/C++ language; however, perl, PHP, ruby and java are descendents that carry on various C language conventions. By not having %n, php gets rid of part of the problem, but it still has %x which can be exploitable under certain circumstances.
printf() and its variants can allow control of the format string if you don't specify it.
Unvalidated user input can supply the format string of their choosing that will lead to a buffer overflow like condition (not a true buffer overflow).
Modern perl and PHP will throw warnings during the development phase so you don't create code missing the format string, if you choose to ignore them, you can have an issue.
Since the site you're referencing allows you to put in any old darn thing to test code, they are preventing creative format strings being entered into
printf() that can eat memory, create a denial of service attack, cause premature program termination or crash the PHP interpreter on that website.
Format String Vulnerability in C