As you correctly say, Powerline Connections (whether Devolo or any other make) are made via a wired Ethernet cable. Hence, as far as the Powerline is concerned, it doesn't matter it the Router is WiFi enabled or not!
Adding a WiFi enabled Powerline Device will enable you to create a local WiFi connection (which will be additional if your Router is also providing WiFi).
My own configuration, FWIW, is a WiFi enabled Router located downstairs, along with a Devolo System extending it upstairs (complete with a Devolo WiFi Unit).
Any "fixed" connections are made by wired Ethernet (whether to the Router or the Devolo), with my portable devices connecting by WiFi to whichever WiFi connection is appropriate.