I would say it is 100% not Unity's fault. As someone else posted, there's a t-machine blog posted and that entire blog has been filled with ECS with that specific term for years. But aside from that it has been a heavily discussed thing for many years in the gamedev scene. There's a wikipedia page on it, showing a bit of history from well before 2017, but you will find most on it on gamedev.stackexchange and gamedev.net. It probably predates the start of Unity in its entirety.

Also, to be fair, they really do refer to it as DOTS -> Data Oriented Technology Stack (granted, it includes more than just that), but I'm not sure if and when they changed that. Their community just seems to keep calling it ECS regardless.