Dietary sources of D are almost always insufficient unless you live on only seafood. Sunlight is pretty much the only viable way to get enough D. Probably not diet related.
You don't actually need to read the article. The summary suggests that all that goes in the email is the amount, and the CC to firstname.lastname@example.org.
Instead, they just email the person they'd like to transfer money to (with the amount as the subject), and CC 'email@example.com.' Square asks the sending for their debit card info, and then sends a link to the recipient, who can transfer the money into any account they want
Most reputable email services offer sender authentication.
The bank doesn't need to sign up for a special program a la the OP's suggestion of Interac e-Transfer. It just uses your debit card functionality.
You don't send account info via email. Read the article, or even just the summary more carefully.
You like dealing with physical mail, and cash? Good for you. Most people don't.
There are many systems like this including POP money. The difference AFAICT is that this does not require bank participation.
Virtually everyone has secure communication to their email provider these days. This is no more abusable than password resetting links that are regularly used for sensitive accounts.
Also I'm not sure if you caught that account numbers will not be transferred by mail.