Is RDSEED a true RNG? Is Intel's RDSEED a true random number generator?
Yes, RDSEED is a true random number generator.

From Wikipedia:


  The entropy source for the RDSEED instruction runs asynchronously on a self-timed circuit and uses thermal noise within the silicon to output a random stream of bits at the rate of 3 GHz [...].


RDSEED is however used as a seed for (software implemented) PRNGs of arbitrary width.