Jordan Rabet, a Microsoft researcher, found an error, used ExprGen, Microsoft's fuzz tool and used Edge's Chakra JavaScript engine. Rabet says he also uses ExprGen on V8, Chrome's JavaScript engine to find CVE-2017-5121.
Ignoring the technical details, Microsoft did not forget the old debt. Rabet pointed out that the fix for the bug he found was put on the V8 GitHub repository, which allowed an attacker to reverse the patching technique and find another vulnerability.
The fact that Google took three days to fix errors on Chromium and Chrome was enough for an attacker to exploit the vulnerability. Because it happened in September, Microsoft had no reason to say a longer error on Chrome no longer exists. Chrome has now updated to version 62.
See also: Invite to download Google Chrome 62 for Windows, Mac and Linux
It is no wonder that criticism in early October about Google's patching process caused Redmond hoters. They did not forget to remind Google that their products were not inviolable fortresses as they thought.