I guess the point is that they need different words for this two-tier system. If they currently use "visa" to mean the kind where you have to show up for an interview and have something glued into your passport, then saying this is not-a-visa makes some sense.
There tends to be a massive difference between the effort required to get a visa, and the effort required to get ESTA.
ESTA roughly as annoying as booking a plane ticket if you don't already have an account with a pre-filled profile, and usually granted instantly.
A visa request, even if done online, can easily cost you many hours filling out forms, scanning documents, etc., and often takes weeks to process.
It's also much rarer for an ESTA to be denied than for a visa to be denied.
There's a reason why passportindex.org distinguishes mostly between "real" visa requirements, and mostly pro-forma stuff like ESTA, visa-on-arrival etc.