Irrespective of debates on what the definition of "open source software is" or who gets to define it, it is very clear that the SSPL is not a FOSS -- free and open source license -- and that's a shame. Sure, open source still means we can look at the source code, but we do not have the full freedoms to use the code for any purpose. You might retort "but I'm not a aaS provider" so my rights aren't affected.
But that's the thing: the erosion of free software rights is never the end, but then beginning of the end. Much like free speech, such rights must be jealously guarded. Need I mention what happens when there's no one left to speak up?
That some users of Redis never contributed back to the project is beside the point: truly free software is free as in libre: if you want thanks for your work, release it as freemium or some other license. But a FOSS license like BSD-3 has always been thankless and the OSI is correct in calling out the SSPL for not meeting the OSI's Open Software Definition's anti-discrimination clause, nor the FSF's zeroth freedom, amongst four.
Free means free. AGPL is free. But SSPL carves out an exception, making it not free. No amount of sweet talking changes this reality.