How does PostGIS project from 4978 to 4326?

Question

I have data in EPSG 4978 (3D Cartesian XYZ using WGS84) and just for kicks, I tried projecting it to 4326 (lon, lat 2D using WGS84). I expected an error but lo and behold, I got a point back.

Here's my query to verify that my point is EPSG 4978:

=>select st_astext(pos_geom) from pos.pos_ts limit 1;
                      st_astext                       
------------------------------------------------------
 POINT Z (1427635.07972 -5672506.90998 2534091.68628)

Attempt at transformation:

=> select st_astext(st_transform(pos_geom, 4326)) from pos.pos_ts limit 1;
                           st_astext                            
----------------------------------------------------------------
 POINT Z (-75.8734015076523 23.5640490271681 -20.0468848655)

Like what?! My first reaction was that the output was probably garbage (-20.04688?) but when I use an external library to do the same transformation, it appears that the lon lat values are correct. So my question is twofold:

1. What is PostGIS doing? AFAIK this behavior, while useful, violates the EPSG standard for 4326. 4326 is a 2D CRS with no Z coordinate in the spec.
2. Please oh please tell me that the Z coordinate is elevation relative to the ellipsoid! I want it to be true but can't find it documented and my external projection method doesn't return elevation.

Answer 1

1. Because it is practical then perhaps you can consider that PostGIS does not do 3D that violates EPSG:4326 but rather 2.5D where Z is some unitless value for height? Z is unitless because the vertical component is not defined by EPSG:4326 but I guess it is meters by default. At least Proj4 has a parameter +vto_meter Vertical conversion to meters https://proj4.org/usage/transformation.html.

2. Proj.4 seems to handle EPSG:4326 as it was EPSG:4979, using ellipsoid height on top of EPSG:4326 that should make you happy. See discussion in https://github.com/OSGeo/proj.4/issues/394.