The context here is calculations about satellite attitude control and motion. There is a frequent need to convert between reference frames, and a vectrix is a clever notation to assist.
The fundamental idea is to represent a frame of reference by a 3x1 matrix in which the elements are not scalars but instead the basis vectors of the frame's coordinate system. That structure, a "vectrix", can be multiplied by [x y z] coordinates resulting in a vector. The idea is developed a bit more at https://vatankhahghadim.github.io/AER506/Notes/1%20-%20Fundamentals.pdf
It makes for an elegant notation, definitely. The question is whether they serve well when you're having a computer crunch numbers, as opposed to when you're deriving equations.