The Merc if equivalently fitted is less than a non-Ti Brompton. It weight would be very close if not possibly less than the Ti Brommie. IF you read their specs and compare the weights of each bike, you may assume incorrectly that the Merc weighs more. It doesn't as it has more stuff fitted on it.

To go superlight you could take the Merc frame and add the Ti Brommie bits. You could strip off the light and dynamo and rack. You could probably get to under 20lbs in this configuration even with the internal hub.