Note this sentence from the wikipedia entry:
"Flightless birds are birds which lack the ability to fly, relying instead on their ability to run or swim, and are thought to have evolved from their flying ancestors.[1"
In the current dino-to-bird thinking, the modern flightless birds are thought to have evolved "from their flying ancestors."
The idea is that dinosaurs evolved into flying birds and then some of those flying birds became land dwellers and lost the ability to fly. There is an element of correctness in this. Current flightless birds did develop from flying ancestors but those flying ancestors did not develop from dinosaurs - they developed from pterosaurs.
There is a additional interesting wrinkle to this, which I will cover in the next post.