The JetBrains team didn't feel like prompting for a Claude UI to go with their project they wrote by hand 8 years ago, I suppose.
Are you really that impressed with stock LLM-generated UI?
Note the JetBrains playground includes features that are actually useful for a code playground, letting you configure compiler version and flags, which is likely more relevant than a faux-IDE UI for the use cases people use playgrounds for. It also supports importing packages, while this doesn't. This doesn't seem like a human even attempted to use it for anything, otherwise they would have realised how lacking it is. Unfortunately it seems like people who prompt slop can't be bothered to dogfood it... or maybe pigfood it?
"From scratch in C" means literally nothing here. LLMs can output C too. It may or may not be safe, efficient C, but writing C that compiles is trivial. Prompting "write it in C" poses no greater difficulty than any other language.