It was added here to guard against race conditions that can occur when calling the underlying native functions from different threads. Calling TypefaceCache::getInstance() or using ReferenceCountedObjectPtr<SharedFontInternal> isn’t the issue here as I think the JUCE graphics code is mostly thread-safe, but obviously there’s nothing we can do if the native methods that are eventually called aren’t.
It might be worth reading through these related threads to see if they can help:
