The page says that the Linux kernel "has no isolation between internal components whatsoever".
When two sentences later it says, "it can be thought of as equivalent to running all user space code as root in PID 1", it is elaborating on "no isolation . . . whatsoever". Specifically, it is saying that the organization of kernel code is analogous to organizing userspace so that all userspace code run as root in a single process.
The author of the madaidans site BTW is a Whonix and open-source developer, so how is it in his self-interest to criticize Linux's security? There is no indication anywhere on the site that the author or anyone else want to sell the reader anything.
When two sentences later it says, "it can be thought of as equivalent to running all user space code as root in PID 1", it is elaborating on "no isolation . . . whatsoever". Specifically, it is saying that the organization of kernel code is analogous to organizing userspace so that all userspace code run as root in a single process.
The author of the madaidans site BTW is a Whonix and open-source developer, so how is it in his self-interest to criticize Linux's security? There is no indication anywhere on the site that the author or anyone else want to sell the reader anything.