This will find Content Items even if those items do not have the values Joe AND true available. This is a search limitation in Picturepark for multi-value fields from schemas. For the search, the values are flattened, so when a Fieldset of a Content Item contains somewhere a value "true" and somewhere a value "Joe" the search finds this as a result.
reviewingContent.reviews.reviewAccepted:true AND reviewingContent.reviews.reviewer.firstName:Joe
The setup uses an underlying schema with two fields.
A workaround is the use of filters for a channel (AND filter and Nested filter, for nested fields), which is also easier to use for the users.