Well did they say that was the final official accuracy? I assumed that was preliminary. If not I agree it would be what's stated. Regardless It'd be pretty dumb to release that without being confident it would hold true.
As far as your experience of pH probes being notorious for going bad and wavering from calibration, that's your experience and also are two different things and not necessarily mutually exclusive. Wavering from calibration can be easily remedied by calibrating regularly, which everyone should be doing anyway. "high quality" lab grade probes are also not all the same and there are several factors as far as installation and operating environment that influence their performance and longevity. Having said that I'm not here to challenge your experiences lol, so let's move past that.
The alkatronic uses pH based titration and has stated accuracy of the same +/- .05 dkH. However in later testing and supposedly expected in most installations it will achieve greater than that like +/- .03. So pH based titration is certainly as if not more accurate. I have found that to be true through my testing of dkh standardized solutions as well as in the accuracy and consistency of the testing of my aquarium.
I'm less concerned with that aspect though, so please focus on this (i dont want to debate ph probes/ph titration accuracy etc): the one pump dosinh different solutions. They may have come up with a way to deal with that and I'm curious to know what it is. That's really what I was expecting to receive as a reply. Someone telling me how/why it's not an issue. I'm curious.