What is the workaround? This video doesn't have anything to do with tagging line items from split transactions, and the steps from @RCV don't solve the problem either. 

This is a critical feature to make tags at all usable. An example: we get one transaction to our bank from our membership software. It might include payments for courses, membership dues, donations, etc, so we split it when categorizing it. If I can't use tags on each line item, that means I can't use tags to track donors or fund targets and I can't use tags to track individual course revenue. A tag on this overall transaction is meaningless