They were not built at the same time. The DEX210 was built 1 month prior after he saw the first pictures of the ER-12 and it looked like it was based off the 210. So naturally he bought a similar buggy to build and race before the ER-12 was ready. He also bought a Losi 8 2.0 4 months before the Agama A8 was released to test that style also. And before that he bought a Kyosho SP2 before the CEN Matrix was released because they were similar.
I would think it would be smart for any pro driver to test the competition before getting their new ride. Especially if they are a close platform.
How else could they say what is different or better or worse and give an opinion at all on the subject?