(Disclaimer: I work on the BigQuery team at Google, but my opinions are my own.)
You're right — our current implementation in BigLake doesn't have full feature parity with the V3 spec yet. We're actively working on it.
The key context is that the V3 spec is brand new, having been finalized only about two months ago. The official Apache Iceberg release that incorporates all these V3 features isn't even out yet. So, you'll find that the entire ecosystem, including major vendors, is in a similar position of implementing the new spec.
The purpose of our blog post was to celebrate this huge milestone for the open-source community and to share a technical deep-dive on why these new capabilities are so important.