This is the same dosage that another study found to be indiscernible from placebo. I suspect a couple things are at play.

The first is that these are treatment-resistant patients, meaning that we already know they are not susceptible to placebo; given that we are effectively comparing combination therapy to SSRIs alone, this means that the SSRI only stage is not experiencing placebo, thus making the therapeutic effect visible by comparison (retrospective comparison, given that there is no control group).

The second is that this is combination therapy. It seems likely that thyroxine is more effective when combined with SSRIs.

Note that this study uses much lower doses than similar studies I've seen, yet had similar results in terms of efficacy rate (roughly 50% responders). I'm not currently certain how the magnitude of effects compares with those studies. Namely, comparing with Pfeiffer et al (350 mcg), Rudas et al (235 mcg) and Bauer et al (482 mcg), and also Bauer et al from 2016 (300 mcg)