From the article:

The data breach started with hackers accessing only around 14,000 user accounts. The hackers broke into this first set of victims by brute-forcing accounts with passwords that were known to be associated with the targeted customers, a technique known as credential stuffing.

From these 14,000 initial victims, however, the hackers were able to then access the personal data of the other 6.9 million million victims because they had opted-in to 23andMe’s DNA Relatives feature. This optional feature allows customers to automatically share some of their data with people who are considered their relatives on the platform.