Anthony from DNSimple here. The ALIAS does synthesize the A record set, but it's the same A record set that is used when a username.github.io domain is resolve

Anthony from DNSimple here. The ALIAS does synthesize the A record set, but it's the same A record set that is used when a username.github.io domain is resolved, which means it should work fine with Github's DDOS prevention. The TXT record is only there for informational purposes and could be removed without affecting the system. Since we have an Anycast network you will also get a result that would be similar to a CNAME resolution, meaning you'll typically get a "close" set of IPs that would be similar to what you would get from the resolution of a CNAME, which ultimately resolves down to A records as well.