I believe you are correct since the algaecidal properties of the Borates don't work until the amount is high enough, at least 30 ppm. In fact, some algae actually like a small amount of borates so the low concentrations certainly aren't helpful (that brings up controversial issues with using Borax for pH control, but I don't think it's a problem so long as chlorine levels are maintained). So yes, I suspect that the Borax is just used for pH control in these kits assuming that something else they have is acidic, probably Tri-Chlor.
Richard
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