I'll try to answer the question.
The prohibition is on banks doing insurance, not on insurance companies doing banking products, so it's a one-way street. The reason for that prohibition being a one-way street is that banks are in the credit-granting business. Credit is a very powerful tool. Credit can be used to coerce a consumer to buy other products based on that very powerful tool of using credit, so the prohibition is a one-way street.
You mentioned Manulife. Manulife does have a bank, and that bank follows the same rules as the big five banks.