The advice is, in essence, to prioritize your mailing list over audiences on proprietary platforms like Twitter and TikTok. This may work for some, but mailing lists seem like they would be really ineffective for a lot of use cases.
Same goes with your own forum, but it's not worth mentioning it almost, since nowadays everyone seems so opposed to a forum. But I believe, a good alternative for a mailing list is RSS (for the people who are tired of signing up for stuff).
I think it also depends on business model and product but mailing lists are fundamentally more targeted than social media. You still need both to some degree.