Apple has confirmed there is a 'membrane' under the 2018 MacBook Pro keyboard.
The 'membrane' will 'prevent debris from entering the butterfly keyboard.' Apple distributed an internal document where it confirmed there is a barrier under the keyboard of the 2018 MacBook Pro.
The Canadian and European internal 2018 MacBook Pro Service Readiness Guide says:
Keyboard and Keycaps
The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism. The procedure for the space bar replacement has also changed from the previous model. Repair documentation and service videos will be available when keycap parts begin shipping.
The U.S. version doesn't mention this, but has a link to an external page, which says:
Caution: The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism. Be careful not to tear the membrane. A torn membrane will result in a top case replacement.
The barrier, Apple calls it 'membrane', was already discovered by iFixit in a teardown of the 2018 MacBook Pro, but Apple has now confirmed it.