No partitionKey for items query in Cosmos DB

I noticed that when querying items in a Cosmos DB database, the code sample on Microsoft Docs doesn't require a partitionKey. Why is that? I can certainly imagine scenarios where we may want to run a query on the whole database but whenever possible wouldn't it be more efficient and faster to limit the query to a particular partition?

Here's the code from the Docs:

string query = "SELECT * FROM products p WHERE p.category = @category"

var query = new QueryDefinition(query)
  .WithParameter("@category", "gear-surf-surfboards");

using FeedIterator<Product> feed = container.GetItemQueryIterator<Product>(
    queryDefinition: query
);

Here's the link to Microsoft Docs: https://learn.microsoft.com/en-us/azure/cosmos-db/nosql/quickstart-dotnet?pivots=devcontainer-codespace#query-items