We were using noninvasive tissue lactate monitoring in pediatric ICU about 10 years ago. I don’t recall what mechanism they were using for this at the time, but I’ve always been surprised that we hadn’t seen this used in sports wearables yet.
I’m guessing this company is using some sort of electrochemical biosensor to detect lactate with their product - Biosensors based on electrochemical lactate detection: A comprehensive review - ScienceDirect
My guess is they are taking advantage of the correlation between sweat lactate levels and serum/tissue levels, and actually measuring sweat levels - https://pubs.acs.org/doi/10.1021/acssensors.1c01009