The article actually said "concurrency." In the case of the article, it's being oversold because it's not that much better than the old one thread per request model. Cross-request interactions are rare, so you're more or less writing the same code, only with async, you have to be careful not to accidentally do something CPU-intensive for 50ms, and if your runtime supports multiple cores (so not Ruby or Python), you still have to be careful with race conditions and locks.

Writing parallel code that isn't clearly a divide and conquer variant is hard. Async doesn't help with that at all; it gives you more efficient context switches.

I personally find threaded code easier to reason about because there can be a context switch anywhere, so you always need to think about locks and coordination. With aysnc code, a block of code without locks and be correct...until you add an await in the middle, and this isn't always obvious.