The 0-day is in a popular software package. The GitHub repo apparently contains a backdoor ability to execute code from the attacker. If I had to guess, this would be the software update functionality here: https://github.com/dbgsymbol/getsymbol/blob/cb4bdedc1a85c308...
"In addition to targeting researchers with 0-day exploits, the threat actors also developed a standalone Windows tool that has the stated goal of 'download debugging symbols from Microsoft, Google, Mozilla and Citrix symbol servers for reverse engineers.'
The attackers used a 0-day but getsymbol is not one.