gentoo is “built from source”, but its package manager, which is wonderful, does all the heavy lifting. it’s a bare distribution, but all the customization is user space stuff. so it’s not a great way to learn “linux”, IMO. a stage3 tarball is already more of a “mini” distro.
Yes, exactly, though you can start from earlier stages too. The process even from stage 1 is well documented. It is all there but you can customize it completely from the ground up. Or, if starting from stage 1, the foundations are there and you have to build it up yourself.
For a newbie (looks to be the intended audience of the article), this gives them a working foundation to start from. They can get progressively more involved with whatever part of Linux they desire, as their experience grows.