Yeah, that blog isn't right, or at least it shouldn't be. (Just because a DNS registrar allows you to set up @ to point to a CNAME, or www to point to multiple CNAMEs on their authoritative servers, doesn't mean that your recursive nameserver will know what to do with that.)
It might be that Namecheap allows you to configure "CNAME's" but actually implements them as ALIAS records or HTTP redirects or something similar and obscures that from the user. Not having a Namecheap account I can't say for certain.
GitHub is correct, the @ record cannot be a CNAME.
Interesting and thanks for the clarification. I see they have (Alias) in parenthesis so it makes me wonder if they are implemented as ALIAS records. This will take some more digging.
Github stated that if you use an "apex" domain your only option is to set A records to their IP but that doesn't seem right.