The purpose of my comment is to get you to think about the implications of following those principles, especially when the modified code had no reason to change other than to meet those principles.
This comment sounds more like you don't want to defend what you did and hope that generic explanations of how to write code will explain why it is OK to modify untested code. I can tell you right now the first advice will be to write a unittest to test the existing code before you refactor.
As has already been explained clearly in the post - it is impossible to write a unit test for the original code because dependencies were instantiated. How does one write a proper unit test for a piece of code that has hard-coded dependencies.
Your assumption that the code was not covered is interesting though because it actually was covered by feature/integration tests. The feature tests still passed after the refactor by the way. The feature tests took the calling code for the State Transition class - passed with concrete validators and data from the database and then verify that after we call the transition end point the actual record in the DB has transitioned as expected. But all of that is completely out of the scope of the original blog.
Your comments have read like someone who is learning about proper software architecture. The end points you have arrived at differ wildly to your initial (incorrect) assumptions. I will not respond to further comments about this now. It has proven pretty fruitless and I once again suggest you brush up on SOLID principles.
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