Generally, a microdose is supposed to be below threshold. In other words, imperceptible. A low dose can be above threshold. This study seems to be testing something most microdose adherents would refer to as a low dose.

Microdose = sub-perceptual dose

Threshold dose = dose at which effects can barely be perceived

Low dose = dose at which effects become definitely perceptible

The people in this study were taking something between a threshold dose and a low dose, given some interpersonal variability.

.5 is definitely a low dose for anyone who has experienced it more than a few times or an actual low dose at least once. Realistically, I'd expect only a first timer could possibly think .5 is imperceptible.