First, it does not yield that. This method would select only mix and plasticizer.
The reason for looking at interactions first is because it is almost always a good idea to include the main effects that are part of an interaction. So, if you keep an interaction, you should keep the main effects that are part of it.
But this whole approach is wrong. It is a form of backwards selection. It doesn't work well. The results will be wrong: The p values too low, the standard errors too small, and the parameter estimates biased away from 0.
See the model selection tag and particularly this thread.
In general, it is not safe to rely on "online resources", unless the author is established. This site is an exception, because answers can be commented on or flagged if they are incorrect.