If they had built their own comment section with Google login, that would be weird indeed. But if you just like Disqus comments (they certainly were hot when I actively blogged in 2012) and one of their login options is Google... I don't see the issue there unless they're really pushing you to use it.
Author said "If a website offers you to sign-in using Google (or any third-party service, say Facebook, Github, etc.), don’t use that feature." But he has Disqus comment section which offers login through Facebook, Twitter and Google. I would say that's little bit hypocritical.
The similar thing is when a politician says corruption is bad and next thing you know he or she is involved in corruption scandal.
Disqus offers you sign-in using one of aforementioned services but you don't have to use that feature. You can make an account normally and sign-in using email/passsword. The author says you should prefer the later, not boycott any service offering the former.
