the bsd-3 license they use is the standard one.
the 3rd clause means that you can not use the names to endorse the product which i think in this case means certified. the way i interpret it is feel free to use the code but include the copyright. at the same time do not label the product a certified or endorsed by the copyright holders which in this case are the OpenThread authors unless you have their permission which probably would mean having to join the group. my guess is as long as the product does not say certified OpenThread or use any of the authors/group names but otherwise works it would be ok. thing is marketing it that way would limit traction in the marketplace. the copyright holders are ok with you using the code they just don’t want you mucking up their good name by marketing it as endorsed/certified unless you go through their process. that’s my opinion based on previous bsd license experience but not openthread. also, i’m not an IP lawyer.