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.