When I tested trying to do it in Obsidian just then, that’s what I found.
I speculated that it may not be possible because Obsidian is not a markdown editor per se, but rather a vault processor that contains a markdown editor. So, from Obsidian’s pov, asking it to open a file can’t work unless it knows what vault it is in. Vaults are unaware of the existence or non-existence of other vaults, so there can never be an assumption that the file will be in Vault X, even if Vault X is the only one that exists. In which case, there must always be an answer to the question ‘Which vault?’ before its editor is able to open any file. And consequently, it’s not something Obsidian will ever be able to do without a change in design. These are only my own presumptions - you may be able to explain why I am mistaken.
That’s my conclusion.
Though, I assume that Obsidian could be redesigned so that Obsidian Editor could operate as a standalone editor, which is automatically triggered when a vault is opened.