It might make it convenient to do those things, but it doesn't make it a database really though does it? The article admits it doesn't have to handle the most complex problems associated with data persistence (getting transactional blocks of data to a storage medium).
Your examples become a problem if your graph can't fit in the RAM of one host. If not that, then you have then distinct graphs to handle, and so a paralellisable problem to be handled by multiple processes. All you have in that case is an abstraction which lets you have a unified interface to all of them. Notably that also isn't relevant as the one in the article doesn't do any of that. It is a graph library.
Your examples become a problem if your graph can't fit in the RAM of one host. If not that, then you have then distinct graphs to handle, and so a paralellisable problem to be handled by multiple processes. All you have in that case is an abstraction which lets you have a unified interface to all of them. Notably that also isn't relevant as the one in the article doesn't do any of that. It is a graph library.