TL;DR: You are correct 2018 and 2017 MacBook Pro and iMac will not DIRECTLY take advantage of WiFi 6, but there is more to the story.
Thats correct, Wifi 6 is all about efficiency in WiFi. In the past the IEEE has focused on improving WiFi by increasing bandwidth for the most part. In WiFi 6 they decided to take a look how they improve it with by making it more efficient.
ie. breaking up the channel into small chunks and send data to multiple clients. Resource Units.
To fully support WiFi 6 you need both hardware and software that's not currently available to the MacBook Pro. One the main features from macOS side (outside of a driver) is support for 802.11r or fast roaming. This is one thing for years that macOS does not support. Its my prediction until the showing of the the next macOS update we won't see WiFi 6 cards on MacBook Pro.
Now in regards to speed and efficiency, in theory if you have multiple devices that are WiFi 6 capable connected to a WiFi 6 supported network, these devices will get on and off the network faster, thus being able give more time for current macOS devices to communicate. This could give a small boost to your client experience.
Hope this helps.