I would agree with you if the Linux RNG was build according to common cryptographic wisdom. But as far as I can tell by reading the article the random sources are compressed in a common randomness pool using CRC. If RDRAND was added to this pool it might reduce the randomness of this pool. If these sources were added by a cryptographically secure mixing primitive this would not be possible. A faster alternative would be to maintain one pool for every randomness source that is added to via CRC and then only mix all pools together with a cryptographically secure primitive when randomness is requested.
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It could be summed up as is: either you have a test that shows that rdrand has got problematic behavior or you shut up.
Backdooring rdrand is of little to no interest given how PRNGs are built.
I would agree with you if the Linux RNG was build according to common cryptographic wisdom. But as far as I can tell by reading the article the random sources are compressed in a common randomness pool using CRC. If RDRAND was added to this pool it might reduce the randomness of this pool. If these sources were added by a cryptographically secure mixing primitive this would not be possible. A faster alternative would be to maintain one pool for every randomness source that is added to via CRC and then only mix all pools together with a cryptographically secure primitive when randomness is requested.