How many thousands do you think Disqus charges its enterprise customers? Also, remember marketing, some tiers exist to get you to buy others.
Still, I have had companies on that tier. :)
And it's not basic, FastComments has a lot more features than the competition, like live commenting and such. I provide direct development and customization support, we support automated migrations from major providers, and it's reliable, covered with around a hundred E2E test suites that run periodically in production. It also has an "enterprise" multi-user/role system and SSO integration.