It’s good to see this much curiosity, and indeed a good question,
Now, for your doubt, the most important thing to understand is, that, Microflow is a transactional activity, so the microflow will start its transaction from the start event. Then it goes to its first activity, in our case that is the start date check, and then if the decision is valid, it then goes to the second activity, and so on….
Now, as soon as the microflow is triggered, it checks whether the start date is selected or not, if it is false, then the flow will go to the end event, and the microflow will end and no activity will be done,
I hope this clears your confusion,
Let me know if you have any other confusion,
Hope it helps!