Maybe it simpler that quaternions (debatable) or simpler than learning all of the concepts you've mentioned separately, but I wouldn't really call it a "simple framework".
I think that simple should be understood here as easy to use. As a crude analogy, the mechanical principles underlying a car with an automatic shift aren't simple, but the abstraction it provides makes driving much easier to learn.
A more mathematically grounded example could be the complex plane compared to the real line: when encountered for the first time, it is not simple in any way, but it offers a clean solution to many (originally real valued) problems (perhaps by the fact that it is algebraically closed?).
NB: the author of this library also made a very interesting presentation of geometric algebra (using this library as vehicle) for SIGGRAPH 2019: https://www.youtube.com/watch?v=tX4H_ctggYo
I agree that if you only need one specific part, than your needs are better met by using just one of the other frameworks. But in my experience that is rarely ever the case. Usually I have to work with at least 3 or 4 different ones. You don't even have to use all of the possibilities for a unified framework to be simpler already. So the word "simple" is meant in relation to the fragmented alternatives.
A more mathematically grounded example could be the complex plane compared to the real line: when encountered for the first time, it is not simple in any way, but it offers a clean solution to many (originally real valued) problems (perhaps by the fact that it is algebraically closed?).
NB: the author of this library also made a very interesting presentation of geometric algebra (using this library as vehicle) for SIGGRAPH 2019: https://www.youtube.com/watch?v=tX4H_ctggYo
It has been posted a few times on HN before.