There won't be any dotted line if the edit was within 10 minutes after posting.
I think you should move this to Beginners & Help and please change your title if you are doing so. Or else, your thread most probably will be hijacked by spammers.
Is this true statement? I thought every edit with no limitation to time shows up like this.
it's true (just saying in case you don't believe MZ).
and you can try it yourself by posting and editing a message multiple times before 10 minutes passed.