You can't implement ECS on top of EC, but if it's possible the other way around, Godot's argument that most people won't need it would seem a bit weak — just let them use fat components on top of an ECS core, and if/when they need the performance there's still room to push it without side-stepping the whole engine.
I’m currently writing and designing a serialization/networking library that I’m using to build an ECS framework on top of regular Unity EC. Why not use dots? Well, I hate the restriction of using only blittables for component data, I want to have access to the already existing ecosystem of Unity packages, and the biggest is that I am designing it for networking first. It’s slow going (I’m a dad and I have a full time game dev job), but I’m making good steady progress. My serialization library currently features composition, quantization, no reflection, and no garbage collection. I plan on releasing the first version under the MIT license hopefully in the next couple months.
I have a prototype that I’ve been building to test my stuff. Check it out it’s only 30s. https://youtu.be/p4v3ZnS2KBM
If you’re interested in the subject of physics and netcode I’d recommend: https://gafferongames.com/post/introduction_to_networked_phy...
If your want a very deep dive and very technical, I’d recommend: https://fabiensanglard.net/doom3_documentation/The-DOOM-III-...
A lot of good resources at the bottom of the pdf too.
Now learning Godot to be able to quickly learn.