I think that statement about "complex systems" being an alternative paradigm to reductionism is a bit unfortunate (articles in Wikipedia have varying quality). I believe they refer to emergence, a concept which they include in "complex systems", because reductionism in its simplest form means that the whole is composed by the parts and emergence says that there are phenomena which emerge from the combination of individual parts and distinct to these. But these two concepts are not orthogonal, see:

https://philosophy.stackexchange.com/questions/4615/whats-th...

The World3 model of the MIT study is just a set of 149 differential-algebraic equations [1]. The output of the model (population, industrial output, etc.) is mathematically a composition of simpler parts. They are saying that the earth is a very complex system but these four or five variables can be explained by these bunch of equations that represent basic processes. To me, that is reductionism.

[1] https://www.sciencedirect.com/science/article/pii/S147466701...