VirtualBox, at least the version I run, cannot do that on its own. You would need to set up the firewall rules on the host. Which is, of course, possible - but not in a cross platform way (linux uses netfilter/iptables, bsd uses pf, windows uses ... I'm not sure what these days, but many users have a 3rd party firewall as well)

It isn't doomed to be insecure, but its security, portability and convenience/usability have a nontrivial tradeoff which is ignored by the original description. If it's portable and convenient, it is likely going to be lacking on the security front.