I don't agree with the recomended method of flushing but I don't think finishing with water is such a bad thing?
It seems logical whatever is in/around the roots before cutting is going to be in/around the plants vascular system, I've tested that it is the case...the uptake travelling through the system it's not been metabolised, in physics water evaporates leaving all mineral behind so it's seem logical no ec mineral no remaining mineral left behind.
Tbh I've cut plenty testers from plants and never thought eewe! that needs flushed but tbf a tester cut from a plant rarely gets much scrutiny?
I grow them until I'm satisfied they're ready then give them water for 3/4 days before harvesting, 4 days was needed when I monitored the sap ec using capillary matt, basically tapped sap and tested the ec at intervals.