He had a separate interest in async but never actually asked for anything to be merged. You'll notice that it is advertised by Thiago as a proof of concept. One of the authors of the previous patch showed up to give a fairly inaccurate summarize of the previous thread, seemingly attempting to dissuade Thiago from continuing. The characterization of Thiago trying to get something added and being stymied seems to have come from the author of that original thread in a comment here:
In Thiago's words, the inspiration for forking seems to have come from the opportunities that were cataloged by Marc Weber. About two weeks after forking he did add the message_T changes to neovim. To say that his goal with that patch was simply to get async support into vim though is to disagree with Thiago's own words on the thread. He was upfront that he wanted to refactor the vim architecture around a different paradigm that he describes as a message loop.
So, who forked neovim and what does this timers patch have to do with forking if the patch authors weren't the ones forking neovim.