I hate that people feel the need to so strongly attach the generic term open source to Open Source InitiativeⓇ OSI Certified™ license's marketing (anti Stallman) propaganda as the one and only true open source. Redis' license is open in all the ways that matter.
OSI has nothing to do with being anti-Stallman. (I think the most anti-Stallman person is, unfortunately, Stallman himself, having done so much to destroy his own reputation.)
Even Stallman would agree that nothing can be open source if the freedom 0, freedom to use software for any purpose, is not provided. SSPL does not grant this freedom.