Yes, your initial summary is my understanding as well. Basically, top-level domains use fairly arbitrary domains as authoritative nameservers (in this case ns-a[1-4].io), and the company that manages all the .io registrations (101Domain) was allowing any arbitrary user to register four of the seven nameserver domains. So a malicious user could have purchased all of them and pointed ALL .io domains to any arbitrary server (for at least 4 out of 7 DNS requests).

It can't really be mitigated at the user level but it's already been mitigated by the registrar. Still though, jfc.