How non-invasive blood glucose concentration measurement work?
This is not a new thing, by checking the articles about non-invasive glucose level measurement, it started in the early 90's.


  Near-infrared (NIR) spectra of the human inner lip were obtained by
  using a special optimized accessory for diffuse reflectance
  measurements. The partial-least squares (PLS) multivariate calibration
  algorithm was applied for linear regression of the spectral data
  between 9000 and 5500 cm−1 (λ = 1.1-1.8 μm) against blood glucose
  concentrations determined by a standard clinical enzymatic method.
  Calibration experiments with a single person were carried out under
  varying conditions, as well as with a population of 133 different
  patients, with capillary and venous blood glucose concentration values
  provided. A genuine correlation between the blood glucose
  concentrations and the NIR-spectra can be proven. A time lag of about
  10 min for the glucose concentration in the spectroscopically probed
  tissue volume vs. the capillary concentration can be estimated.
  Mean-square prediction errors obtained by cross-validation were in the
  range of 45 to 55 mg/dL. An analysis of different variance factors
  showed that the major contribution to the average prediction
  uncertainty was due to the reduced measurement reproducibility, i.e.,
  variations in lip position and contact pressure. The results
  demonstrate the feasibility of using diffuse reflectance
  NIR-spectroscopy for the noninvasive measurement of blood glucose.



1993 - Noninvasive Blood Glucose Assay by Near-Infrared Diffuse Reflectance Spectroscopy of the Human Inner Lip


There are multiple approaches, mostly spectroscopy, but dielectric properties (e.g. conductivity) can depend on glucose levels too. Saliva can be used to measure glucose levels as well.


2014 - Non-invasive glucose monitoring using impedance spectroscopy
2014 - Broadband Tissue Mimicking Phantoms and a Patch Resonator for Evaluating Noninvasive Monitoring of Blood Glucose Levels 
2014 - A paper strip based non-invasive glucose biosensor for salivary analysis


The details of each technology you can find of wikipedia.