Include testing, writing test cases for both positive and negative use cases and making sure application runs as expected.
If required, we would need the resource to switch hats and work on salesforce administration.
A salesforce QA resource is expected to have SOQL knowledge as they would need to query records and make sure the data is persisted into database as per the expectations.
For example, there are so many instances where there is no front end but data is updated through integrations.
In those cases we would need someone who can query data and verify as a part of testing.
SOQL is a very basic thing for a salesforce resource at any level.
Salesforce admin certifications do include SOQL questions too.