Sure, that's true. But counting on 0.00000 ppm phosphate salt water to reduce phosphate is also not very effective unless the changes are huge.
Starting at 0.05 ppm, a 10% change only drops it to 0.045. That drop is substantially less than you probably fed the tank that same day.
And that assumes there is no phosphate bound to rock and sand. Even a 100% water change with 0.0000 ppm water does not eliminate phosphate. Starting at 0.2 ppm phosphate and doing a 100% change might only drop it to 0.1 ppm due to the large reservoir on the rock and sand.
I don't doubt it may be significant in some cases, but that depends on what you are feeding, not what you are measuring or the methods used exporting. If you barely feed anything, then yes, it may be large. If you feed a normal amount, then I don't see how it is significant at all, especially over time. You just have enough export or consumption to balance all the sources in your system. As I noted, 0.05 ppm used to do, say 30% water changes over the course of a month only contributes 0.0005 ppm daily, which is much less than you probably add in foods each day.