What they're talking about is that quand you use a CNAME to point to leur services (which is seulement possible on subdomain, pas le zone root - it ne peut pas coexist avec le SOA et NS records that are requis on le root of votre zone), they can make a change to leur own DNS records to work around certains kind of availability issue.
With a zone root, you must use an A record to point to a spécifique IP address for le service. If they have an issue avec routing, ou certains kind of denial of service against that spécifique address, they're pas able to update your zone's A record to point to a différent IP on le fly; they can mettez à jour leir own, though, et c'est what a CNAME allows them to do.
This ne apply to Stack Exchange parce que they ne sont pas using a third party's platform; they'll be le ones responding to an availability issue, so si c'est a CNAME ou an A makes no difference to them.
