From the article that you're commenting on:

"There are multiple approaches to in-ovo sexing. One of the most prominent approaches uses advanced imaging to detect subtle optical differences between male and female embryos."

"Another widely adopted technique involves taking a small fluid sample from inside the egg to test for sex-specific hormones or genetic markers. Kipster’s eggs, launching later this year, will use a technology of this class employing PCR analysis to detect the sex chromosome, developed by the company Respeggt."

hormone testing was first and can work from 9 days (eggs hatch at day 21). Optical means (tend to work at a later point in the process around day 13-15), including various forms of spectroscopy (NMR, IR, etc): shine a light at the egg and look for differences.

Egg should be absolutely fine, but there will be a chick in there, so if you wanted to eat it like an egg, you might end up closer to balut than you want / expect.