My understanding is that the tests that Theranos actually ran on patients' blood was actually based on drawing blood from the veins, not the finger-prick test. While Theranos did want to do the finger-prick tests--and it wouldn't have worked for the reasons you mentioned--that they didn't get them working meant they couldn't get certification for actually running those tests. So they ran the tests they did get certification for (which was using venous blood draws), for the most part, but even then, they were running tests in ways that violated the procedures they were supposed to be using.

(I do realize that keeping track of how precisely Theranos was lying can be frustrating, since they were doing multiple levels of lying here.)