It would be nice if specific offending portions of the codebase were highlighted. As of now, it’s hard to see why one should use this fork. Also, since the source is available, anyone can just compile a past version of vim.
I would expect a decrease in code quality in a specific part of the repo or at least a quote/link to a changelog stating that generated code is being used as part of the fork making its case.
If they are correct and things must be done, they aren’t providing the evidence. That’s not the same as saying there’s no evidence or reason to take this stance. Do you see the distinction I am making?