1行足してすぐ消す編集をしてみました。おーなるほど！
$ echo -n hello world > noeol.txt
$ cp noeol.txt vim.txt
$ vim vim.txt
$ diff -u noeol.txt vim.txt
--- noeol.txt 2019-05-13 19:55:06.000000000 -1000
+++ vim.txt 2019-05-13 19:55:58.000000000 -1000
@@ -1 +1 @@
-hello world
\ No newline at end of file
+hello world
$ cp noeol.txt vim.txt
$ vim "+set nofixeol" vim.txt
$ diff -u noeol.txt vim.txt
<違いなし>