My question is short and straightforward: Can be considered "machine learning" a subarea of "expert systems"?
That depends on your definition of an "expert system". Searching around, many definitions include a clear seperation between 'knowledge base' and 'inference rules', and machine learning most definitely does not always make this distinction.
In fact, unsupervised learning has no knowledge base whatsoever, and finds naturally occurring patterns in just unlabeled data.
I would see it the other way round: expert systems is a part of machine learning. Reason: using a neural network, training it means learning. However, a neural network is not an expert system (unless you stretch the definition of an expert system very much). A neural network does not use inference, for example.