Tiago Forte has been working on these ideas for many years in the open on his blog (Fortelabs, the one OP posted) and Building a Second Brain is the course he built atop that. From what I have seen of it, its original work and some synthesis of past ideas, just like most things.
It's weird to see you do these quick Google searches and speculate its plagiarized without any substance to back your argument or awareness of the work he's been doing for years now.
This is one example of me finding some severe plagiarism where someone makes an entire living off of it: this Language Master "course" (https://www.languagementoring.com/courses/master/order/) which costs 990 EUR is basically this guide, "What do you need to know to learn a foreign language?": https://www.wgtn.ac.nz/lals/resources/paul-nations-resources...
Basically, the entire basis of her course is that above guide, but she creates video lessons for each section of the above linked guide with worksheets for each section (which you fill out to the videos). The only content that she adds that is substantiative in terms of work is language learning resources for X Y and Z languages, in terms of inputs/outputs.