For varying definitions of "open source" (a considerable amount of functionality is offloaded to binary blobs in many models).

In terms of duplicating effort, in the page linked it specifically states it's based on Cerowrt, which is in turn based on Open WRT (which is actually FOSS, instead of just in name).

More importantly, it doesn't say "...Help Us Build an Open Wireless Router Firmware" because that's not the goal of this project. As you clearly pointed out yourself, and as they implied in the post they made, that's already been accomplished multiple times over. The goal is to make a router designed for the Open Wireless movement, which is why, in the first sentence, they link to https://openwireless.org/