I tried editing the question you are referring to add four spaces, and my edit has been accepted.
I then reverted the edit I did by removing the four spaces I added; as I did that in the grace period, there isn't track of the edits I did.
Once you get the privilege of editing every post, you can add a single character, or add 6 spaces to a post; it's just the users who don't have the privilege of editing every post that have those restrictions for their suggested edits.
The message you have seen could be the one saying that somebody else already edited the post you were editing, which appears even in the case somebody else suggested an edit. It happens also when you started to edit the post, but you clicked on the button to save the changes after a (suggested) edit has been completed.
Looking at the revision page for that question, I notice that the OP edited the post at 12:56:55 pm, while your edit was done at 12:56:47 pm. This means the OP started to edit the post after you started, but before you ended your edit.
In such cases, Stack Exchange gives you a message; the usual one I see is the following one:
This post has been edited 1 time since you began. Your edit can only be saved if it is more thorough than the currently saved edit.
The message appears when I am editing the post, on the bar at the top of the page, in the same position other notifications are shown.
I am sure you can edit a single character in a post, as that is what I have done many times, in Stack Exchange sites where my reputation allows me to edit any post. Your reputation on Stack Overflow is higher than the reputation required to edit any post, and that allows you to change even a single character, or add a single character, whatever that character is a space, or not.