De :help swap-exists
:
D Delete the swap file. Use this when you are sure you no longer need it.
For example, when it doesn't contain changes, or when the file itself is
newer than the swap file.
On Unix this choice is only offered when the process that created the
swap file does not appear to be running.
Então, parece que o Vim acredita que outro processo do Vim ainda está sendo executado ativamente no arquivo.