On the Thread Test Harness Software Installation and Quick Start Guide there is no mention regarding how to certificate a Border Router.
Always it is talking about the DUT, but it doesn't mention if the DUT is an end device, or a Border Router. We are not Contributor, or Sponsor in the Thread Group, so we don't have access to Thread Test Harness Software.
On OpenThread documentation:
For example, to build the CC2538 platform with DHCPv6 for use as a DUT in certification tests:
make -f example/Makefile-cc2538 BORDER_ROUTER=1 COMMISSIONER=1 DHCP6_CLIENT=1 JOINER=1
When OpenThread project claim that is Thread Certified on CC2538 device as Border Router or Full End Device, what does it mean exactly?
Thread devices may support a varying set of capabilities, including:
The Thread Certification Test specification includes a set of tests for each of the capabilities above. The set of tests applied to a DUT are selected based on the device's stated capabilities.
OpenThread has achieved certification on all of the capabilities above (and as listed on the certificate you linked).