I heard something similar to this too more in regards to co-ordination and motor skills specifically though, if they don't get crawling properly down pat. It's not how long they crawl for that's the issue it's whether they do it properly to use both sides of their brain in sync.

As for the thing with walkers. I think it has to do more with weight bearing and their muscle development & their hip placements. the seat in the walkers tend to hold all the babies weight in their hips and they usually have to 'tip toe' to walk anywhere or touch the floor when parents start putting them in.