That does remove a large class of attacks, but there are still IDN homograph attacks possible with single-script domains, if they happen to use only characters

That does remove a large class of attacks, but there are still IDN homograph attacks possible with single-script domains, if they happen to use only characters that can be approximated in another script without mixing scripts. For example, if you registered να.eu ('να!' is a Greek interjection), you could be spoofed by the Latin va.eu. ICANN's approach goes a step further and disallows registering a new domain if it is confusingly similar to an already registered domain. To allow that to be checked automatically, they use a set of quasi-equivalent pairs (e.g. Greek 'α' <-> Latin 'a') to determine what counts as confusing similar.