I had the same question and ended up creating a support ticket with AWS.
This was their response:
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When creating materialized views from Zero-ETL tables across databases, users need both:
SELECT permission on the materialized view in the target database
SELECT permission on the source Zero-ETL table
This differs from regular cross-database scenarios because Zero-ETL maintains a live connection to the source RDS MySQL database. The additional permission requirement ensures proper security controls are maintained across the integration.
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This means that the documentation you are looking at for permissions is not valid for zero etl source.