Subdomains follow the usual domain requirements:
Use only these characters:
- (not as a starting or ending character)
Label must 63 characters ("octets") or less and names must be 255 characters ("octets") or less.
For more information, see RFC 1035.
Putting it simply, Google or any other name servers are going to have to recurse to .et nameservers in order to access any information in them; if they collectively block Google, then any URL-based requests will fail, as name resolution will fail.
This is an overly simplistic answer of how, but it’s possible.