On a day when you want something toasty, your hand goes to the darker packet. "Dark roast" on the front feels like a promise of a stronger aroma. And the room does smell stronger a little way into roasting than at the very start, so "the darker, the toastier" fits what the kitchen tells you.
Does toastiness keep increasing the deeper the roast goes?
Not in the Kochi trial. As seen in chapter 2, the two pyrazines measured in that trial as an index of roast aroma came out higher in the light roast than in the dark in first-flush tea and in karibancha (there was no difference by roast level in second-flush tea). What settles toastiness is not depth alone but the combination: at what temperature, and how far.
Roast deep enough and the roasting temperature stops showing in the components
What is being compared is the line-up of components in the 2020 Ishikawa experiment, where stems were roasted at 180°C, 200°C and 220°C, each from light to deep.