Postgis ST_Transform(point,4979) doesnt return the correct elevation for use in cesium?

CesiumJS
1

I am trying to understand either what is wrong with my approach here or my understanding using 4978 ECEF coordinates from postgis in cesium.

The following query

SELECT 
    ST_X(ST_Transform(ST_SetSRID(ST_MakePoint(longitude, latitude, elevation), 4326), 4978)) AS x,
    ST_Y(ST_Transform(ST_SetSRID(ST_MakePoint(longitude, latitude, elevation), 4326), 4978)) AS y,
    ST_Z(ST_Transform(ST_SetSRID(ST_MakePoint(longitude, latitude, elevation), 4326), 4978)) AS z
FROM 
    (SELECT -121.93615138888889 AS longitude, 37.365277777777777 AS latitude, 0 AS elevation) subquery;

returns:

"x"	"y"	"z"
-2684803.3918930604	-4307256.836192265	3849690.991410601

The point doesn’t sit flush with the terrain as seen here: