I think there's a subtle difference: you want to create mutual exclusion around specific operations using that string. However, it's at least conceivable that something else uses the interned instance of that string, and thereby creates contention.
Now, if your string is sufficiently unique, the chances are relatively low, as long as you don't leak a reference to it (an object, or explicit Lock only has one purpose, so that's less likely).
Still, it's basically mysterious action at a distance. Whereas using the ConcurrentHashMap, it's very explicit action at a distance. Granted, it does require explicit JVM linkage, which is a cost.
Now, if your string is sufficiently unique, the chances are relatively low, as long as you don't leak a reference to it (an object, or explicit Lock only has one purpose, so that's less likely).
Still, it's basically mysterious action at a distance. Whereas using the ConcurrentHashMap, it's very explicit action at a distance. Granted, it does require explicit JVM linkage, which is a cost.