> color change [is] picked up by a set of photocells in the dongle, and the results are then sent to an app
The logic to discriminate a color change could have been handled by a 75-cent microcontroller. Add a cheap display or a couple LEDs to show the test result, plus a cheap AA battery, and you have a self-contained gadget for less cost than creating the smartphone interface.
Pregnancy tests and all sorts of sophisticated chemical test strips don't involve a smartphone.
I'm thinking that the smartphone hookup is either:
(1) a way to add future functionality like maintaining a database of the tests or uploading test results to somewhere (with all the privacy implications and risks of those), or
(2) a marketing ruse since we all know that a smartphone app that does X is way cooler than a old-fashioned self-contained gadget that does X.
An an example of how easily people fall for coolness of having a smartphone do something that doesn't actually need a smartphone, check out this comment to the original article:
"I have been resisting the smartphone trend for years because they are expensive and there was no unique use for them: everything a smartphone did I ALREADY own a gadget for. This changes the game. Now I want a smartphone."