This colander was definitely not CNC machined, it was injection molded. Without being able to inspect the part closely, I will guess that the shutoffs (the open spaces between the plastic ribs) were formed by a retractable feature in the mold which presses against the mold core during the injection process, and then is pulled away from the core when the mold is split, so the part will not hang up in the mold.
The other way to do this without a mechanism in the mold as described above is to cleverly shape the halves of the mold in such a way as to form the shutoffs naturally as the two halves of the mold are pressed together. This is a tricky thing to accomplish because it creates an extremely complex parting line between the mold halves which greatly increases the chances of creating flash, where small amounts of plastic squeeze inbetween the shutoff zones and leave thin webs of plastic that close off the shutoffs.