It's not really about the flawed test. It's known to be a flawed test. It's about the flawed system.
By 1978, the Department of Justice had determined that field tests “should not be used for evidential purposes,” and the field tests in use today remain inadmissible at trial in nearly every jurisdiction; instead, prosecutors must present a secondary lab test using more reliable methods.
The Department of Justice had already stated decades ago that these tests should not be used as evidence to convict someone. If the defendants were given that information along with the information that a defense lawyer can help them, they might not plead guilty to a flawed drug test.
But this has proved to be a meaningless prohibition. Most drug cases in the United States are decided well before they reach trial, by the far more informal process of plea bargaining.
This puts field tests at the center of any discussion about the justice of plea bargains in general.
This is more about how the system fails by not communicating rights to people appropriately.