I don't want to get into a philosophical debate about what is "open source," but I have no issues with how Sentry advertises and documents their licensing or in how their custom terms are actually written.
Commercial use of a copyrighted work in a way that directly competes with the purpose the work is being sold for is almost certainly not fair use. Hell its almost the definition of what something not covered by fair use would be.
IANAL
Allowed:
Deploy your own Osso instance and use it in applications you own and operate.
Not allowed:
You use Osso to start a business where your primary business is in helping other companies implement SAML SSO in the applications they own.
If that's not what our license says then we will change the license text.