WARNING: Node v10.8.0 is not tested against Ember CLI on your platform

I successfully installed Node 10.8.0, npm 6.3.0 and ember-cli 3.3.0 on my mac:

$ ember -v
ember-cli: 3.3.0
node: 10.8.0
os: darwin x64

$ npm -v
6.3.0

However, when I run

ember serve

on the ember demo project (super-rentals), I still get this error - even though the link says that the combination of ember & node is fine:

WARNING: Node v10.8.0 is not tested against Ember CLI on your platform. We recommend that you use the most-recent "Active LTS" version of Node.js. See https://git.io/v7S5n for details.

Any ideas what I am doing wrong ?