From the blogpost:

“In reaction to ongoing advances in quantum computing, we recently updated our quantum threat model and established 2029 as the target date for Google’s Post-Quantum Cryptography (PQC) migration. Accelerating the timeline means we must hasten the development and deployment of quantum-safe digital signatures, particularly for public-key certificates. While deployment of quantum-resistant key exchange solutions is progressing rapidly to protect data in transit, digital signatures present complex challenges that must be urgently addressed. This post explores these varied technical hurdles and the opinionated paths we are taking at Google to provide clarity and move the PQC transition forward. […]”