From the article: "PGO was initially introduced in M53 for Chrome on Windows using Microsoft Visual C++ (MSVC) […] In M85, we are rolling out PGO on Mac and Win

From the article: "PGO was initially introduced in M53 for Chrome on Windows using Microsoft Visual C++ (MSVC) […] In M85, we are rolling out PGO on Mac and Windows using Clang." So it sounds like they've been using PGO for a while using MSVC, and now they've started using PGO with Clang.