It is true.
I am an old-time spammer. I sent millions of emails back in the day.
Double opt-in email marketing has been around since the 1990s. Few email marketing list owners asked potential subscribers to confirm the subscription.
It was and still is easy to collect email addresses. There are email harvester apps that scrape websites, search engines, forums, etc. for email addresses. The harvester will search for email addresses using keywords. (I still have my app.)
But I saw the light and reformed. Well, actually, I read the CAN-SPAM Act when it came out in 2003.
The volume of and complaints about Unsolicited Commercial Email (UCE) increased to the point where the federal government stepped in.
The law was not as strong as the anti-spam faction wanted because the law still allows UCE. The CAN-SPAM model is "Opt-Out." In other words, the subscriber is in until they opt out.
At the time, email marketers recognized that, yes, they CAN still send SPAM. They just had to follow the legal requirements for UCE. https://www.ftc.gov/tips-advice/business-center/guidance/can-spam-act-compliance-guide-business
The U.S. Federal Trade Commission published a Compliance Guide for Business. The Guide identifies 7 main requirements.
In 2018, the FTC considered changing the requirements of the Can-Spam Act. In early 2019, they decided to leave the law as it is.
Using Double Opt-In is an industry best practice now.
MailChimp erroneously states the law requires double opt-in. As a service provider, they can establish their own terms of service for users. They did not need to say it was a law. Note: I am not picking on Mailchimp. I checked MailChimp by chance. I expect others to say something similar.
Each email marketing service (EMS) provider sets their own terms of service. Because countries have different UCE laws, it is easier for EMS providers to default to requiring double opt-in.
One of the reasons why EMS providers require double opt-in because of the impact on their delivery rates. For some reason, only Google knows, email from GetResponse goes straight into my Gmail spam folder.
For more about email marketing and list building, go to https://homebusinessforveterans.com/email-marketing-email-lists