The article doesn't say, but if it used an OpenStreetMap-based routing engine (which you can self-host because, y'know, open data) then the directions might misalign to the display layer which is Apple Maps. Therefore it must send your directions request to Apple. Which I guess is fine, they'll just see "some DDG user" requesting certain directions and it's definitely not the worst company they could be working with, I'm just a bit surprised that DDG went with Apple Maps instead of self hosting.