At this moment in time, it's as designed. The option just controls whether or not the links will be automatically converted to unfurls or not. It doesn't prevent existing links being unfurled, nor does it prevent using the unfurl="true"
option manually. This is similar to how media embeds work -- the option for this controls whether or not links are converted automatically, it doesn't prevent you from using [MEDIA]
tags.
If you feel there needs to be a hard switch to disable the feature entirely, you would need to post that as a suggestion at this point.
I have clarified the wording of the option, at least, to infer that it relates to the "automatic" process.