To group variants by a parent product without using a Handle column, you can use the ID column if you're updating existing products. However, if you're creating new products and don't have an ID or Handle, it becomes challenging to group variants without a unique identifier.
In such cases, it's crucial to have some form of consistent identifier across all rows that belong to the same product. If you don't have a Handle or ID, you might need to reconsider your import strategy to include one of these identifiers to ensure proper grouping.