This is really not an introduction to CMake. It is more like a collection of the author's opinions about CMake, barely categorised into sections and some quite contentious.
An introduction would be example driven, starting with a very short but complete example:
cmake_minimum_required(VERSION 3.1)
project(FooAndBar)
add_executable(Foo foo.cpp)
add_executable(Bar bar.cpp)
It would explain projects and targets (noting the different meaning of “project” to some IDEs including Visual Studio: projectsolution and targetproject). Then you would progressively build from there. Start by adding a dependency such as protobuf to show find_package() and target_link_libraries(), showing both the new protobuf::libprotobuf target dependency and the old-style ${Protobuf_INCLUDE_DIRS} and ${Protobuf_LIBRARIES} variables. Then make some shared code between your two executables into your own library, discussing shared vs static. I would not even mention variables until after this point (even though ${Protobuf_INCLUDE_DIRS} already is a variable).
In other words, an introduction should be top-down and pedagogical. This is the exact opposite of that: picking through a CMakeLists.txt from the bottom up, one line at a time, before you even know what the point of it is.
Perhaps I misread the title: I read it as “introduction to CMake [but using modern techniques]”, but maybe it’s “introduction to the modern bits of CMake [assuming you already know the old stuff of CMake]”. But even that could be example driven, admittedly with more effort: start with an old crusty CMakeLists.txt with plenty of bad habits and make it better one step at a time. Or have lots of little CMakeLists.txt with one bad habit at a time, and fix each of those.
I am not convinced by some of the recommendations it makes either, although I think there will always be some disagreement about some of these things. I have already given my view on the “cmake_policy” atrocity (which is the very first thing in “Introduction to the basics”!) in another comment here. And in the examples there are workarounds for old versions of CMake that don’t have targets for e.g. find_package(boost) by creating interface targets using the old _LIBRARIES and _INCLUDE_DIR variables. These are very neat but not very accessible to CMake beginners. It would be much simpler to put up with using the old variables, or commit to increasing your minimum supported CMake and forget them entirely.