The two technical reasons listed by the author:

(1) Cookie prolongation to subdomains as a safety measure (I don't buy the performance issue -- if cookies harm your performance, you misuse them)

(2) Cannot use CNAMEs on @ (the domain itself) in the DNS.

I completely buy (2) because it can make certain setups less flexible. And I don't want to give the NS to some CDN (as it is required (is it?) when using Cloudflare).