Did you rebuild after you changed the value? (Enable a CDN for your Discourse, which is linked to in the yml file you edited says to rebuild after the change.)
No, what needs to be done is to do a rebake of the posts to fix the references to the old CDN. See Disable CDN causes old images with CDN URL can not be shown - #2 by zogstrip for two ways to do it. The simpler way is:
# ssh into server
cd /var/discourse
./launcher enter app
rake posts:rebake