Earlier quoted context omitted.
Don't expose MySQL databases to the internet. Just don't. Stick an API layer in at the very least with key based auth, and only the bare minimum capabilities allowed for the user. That said, if you'd read the article you'd see that so far only unsecured MongoDB, Elasticsearch and Redis installations are being attacked so far.
It's not strictly necessary, as long as there are no known vulnerabilities, and you use sufficiently hard password. However, it's highly recommended you never expose such services to the public, or at least limit allowed IP ranges.
That's really not something you should be gambling on. Assume the worst, and architect based on it.
Limit what can be exposed where / how (e.g. as you say, "at least limit allowed IP ranges", but to me that's absolute bare minimum)