It starts with the string " touching " the fretboard. Not enough to choke the string though, and then players fingers do the rest, so it's really a combination. So yes, i'd agree with you in the most part that a player can dial it back. Actually, some fretlesses dont sound like a fretless and some do.
If the player doesnt " force " the mwah with technique with such fretlesses, then one wouldn't know it was a fretless. Some you can tell its a fretless just plucking open strings.