The ops is not saying that the private keys are shared online, there are ways you keep documents in your email for future use such as saving them in the draft folders. I have always save my private keys in my email drafts because I have strong security on my email such as the authentication of code before logging in and that is done using either my phone number or google authentication app.
And where do you think the drafts are stored, if not online? Everything in your email (inbox, outbox, trash, drafts, etc) is copied to an unknown number of servers across the world. How do you know all of them are secure? How do you know all the employees are trustworthy? Just because your account itself wasn't broken in to, doesn't mean that all the data inside it can't be accessed by other means.
Additionally, although 2FA is obviously a good idea to have on all your accounts, having 2FA doesn't make your account immune to being hacked by any means.
Storing private keys online, especially in plain text format, and then double especially telling people on a public forum that you do that, is terrible security.