The author built the stool with 2x4s and then had a CNC engrave the top. Pretty slick looking. One thing I would have done differently. Since a CNC did the engraving, why not have it also cut it out? Looks like the author is using a shapoko too.
http://wilkerdos.com/2015/08/diy-simple-step-stool-with-cnc-carved-top/