It's specifically archetype based ECS that's been patented. Like (seemingly) everyone else in gamedev, I've implemented ECS multiple times over the past 10 years. The last time I did it was in Rust, trying to be the first to use Rust's awesome parallel processing constructs to make an automatically parallel ECS, I got distracted halfway through and then specs beat me to it.
I'd never seen or heard of archetype based ECS before I first saw Legion and I was very impressed with it. I don't know where they got the idea but it's very worrying if Unity legit came up with this first. I'm not super up to date on what's happening inside AAA studios, but what I've seen in production is that game studios usually manually group components to optimise performance. The automatic grouping in archetypes is definitely not mentioned in any tutorial or game engine architecture I've ever seen before Legion.
I remember reading about them in my CS studies, and that was pre 2011.