Can you back up this claim? I think it's not true and authors of both projects respect each other. They simply have different views regarding backwards compatibility etc.
I think 'hostile' is a bit harsh. I'd say 'infinitely reticent to large changes'. All the neovim authors wanted, was a list of blockers needed to merge their async patch. Bram would never give them that, always coming up with one more change. I've seen this pattern of infinite reluctance before, especially when communities depend heavily on one maintainer. its not about async and the specific semantics of the patch after a certain amount of time, it's about deeper issues like project vision, control, and perhaps trust. https://groups.google.com/forum/m/#!topic/vim_dev/-4pqDJfHCs...https://geoff.greer.fm/2015/01/15/why-neovim-is-better-than-...,
All they wanted was to fork the project to "clean up" the code in ways they wanted, manage the community themselves, etc. It wasn't like all their patches were flawless and Bram just wouldn't accept them.
Just dumping any old code into a project the first time it's demanded is a sign not of enlightened maintenance but of gross negligence. It's good if there's a lot of feedback on patches, it's not some kind of aggression.
Yeah, not going to happen. ViM's author is remarkably hostile to the efforts of NeoVim and the ideas it brings forth.