Use of a decision making methodology is different from making a decision. Your example of creating a matrix could be a Next Action, involving lots of decisions, and the result of the matrix is an output which still needs a decision- whether to accept the output or not. None of these individual decisions can be Next Actions.I cannot agree. There are formal decision making methodologies/processes/techniques. Here is the Wikipedia list: https://en.wikipedia.org/wiki/Decision-making#Individual .
For example you can create matrix with all the options (columns) and all the factors with their weights (rows) and "calculate" your decision. It is a physical action that perfectly matches the GTD's Next Action definition.