Lakebase is referring to the fact that in addition to disaggregated storage s3 is authoritative storage for older data.
Since data is on s3 (or lake) you can perform direct to s3 type operations like data loading, reading this data by engines that are not Postgres and more
But if you say "Disaggregated Storage on S3" then you have the flexibility to change that to "Disaggregated Storage on FOOBAR" to avoid confusion.