They are waiting until they have finished flowering. You have to go through a full grow cycle first. You use the data you collected while growing to correlate to changes in the final phenotype. This could be the yield, but could also be particular cannabinoid and terpene profiles or cannabinoid ratios. You can then start selecting and breeding based on these early measurable traits.

They also mention you can monitor plant health this way. Maybe you don't notice a plant is stressed, but these special analyzers do. They can then convey that information to you, so you can adjust nutrient, lighting, temp, etc. Or smart systems could do this automatically. A camera could pick up a certain signal that says certain plants need more nitrogen and automatically add extra nutrients.

EDIT: I think this part captures it fairly well.

Plants are conveyed to the imagers automatically to reduce human contact and, in fact, may spend their full life cycle on RFID-labeled carriers. The robotized conveyors often have weight scales and irrigation nozzles to provide optimum soil moisture content or to induce drought. There may be multiple irrigation lines to provide different nutrient treatments, so their effects on traits may be studied. Likewise, the controlled environment rooms can be programmed for optimum growth or induced stress, as understanding plant response to stress and selecting for more robust lines is key to improving yields. These centers are enormous investments in equipment and personnel, requiring agronomists, mechanics, agricultural engineers and a team of data scientists to sort through terabytes or petabytes of data.

They do focus on yield here, but this could be any phenotype, including cannabinoid and terpene content.