The client gives the server a list of id’s (how is this list of IDs determined?) of shares they want to validate, I understand this piece. There is just no guarantee that the pool is providing accurate information. This is the same as using an API to query for shares.
What stops a pool from providing inaccurate or misleading share data? or omitting shares when requested?
Edit: Inaccurate or misleading shares would fail the Merkle inclusion validation piece. but omission remains un answered.