It sounds like the girl herself is content and in no need of any pressure to be other than happy in herself. Making her self-conscious about her natural way of being in the world won't add to her interest in being with strangers.
It is ironic that children are taught not to talk to strangers, but when they have no interest in talking to strangers, it's a problem. To a child, and frankly in reality, there is no difference between a total stranger who is 42 and a total stranger who is 5. If talking to strangers is dangerous or wrong, it doesn't matter what age they are, does it?
Do you know that the parents of children who can't (or won't) play on their own are also being told 'this is a problem,' too?
Not everyone in the world is Robin Williams --thankfully. Not everyone is Howard Hughes, either. There is room for a range of personalities, and not everyone needs to be the same to be successful.
Besides, it's age-appropriate to be wary of others. She'll probably grow out of it in her own time.