To my understanding, it's a new, nonstandard thing invented by DNSimple that lets you specify your base domain as a reference to another domain — basically, it's like a CNAME where an A record would normally be required.
For example:
If you set up subdomain.yourdomain.com to be an A record to to 10.0.1.1, you can also set up additional A records, AAAA records for IPv6, and even MX and NS records for subdomain.yourdomain.com.
If you set up subdomain.yourdomain.com to be a CNAME, you can't have any other record types for subdomain.yourdomain.com.
ALIAS records exist because you can't CNAME your root record - the root of your domain has to have at the very least an SOA record, and NS records are going to be needed for the domain to actually resolve. Since those record types exist at the root of your domain, and since CNAME's can't coexist with other record types, you can't use CNAME's at the root of your domain.
Accordingly, the ALIAS record really is only useful to redirect the root apex of your domain somewhere else.
(Just my two cents.)
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.
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.