My Sago Palm (Cycas Revoluta) has weird-looking clumps in its roots. Is it a Phytophthora root rot infection? If not, what is it?
No, it is not. Those are nitrogen fixation nodules, similar to what alfalfa roots do. It is the result of symbiosis with a soil bacterium.
With root rot the roots die and things get a little stinky. Pinch an affected root with a thumb and forefinger and tug - the epidermal tissues will slide right off the xylem or woody core.
Agree with Jim Young's answer, point given, but would add these are referred to as coralloid roots, and are common on Sago palm, image below
https://commons.wikimedia.org/wiki/File:Cycas_revoluta_coralloid_roots.JPG