Get ArcGIS to recognize ST_POINT

I don't have an SDE to play with. I think you would have to do something like this:

With point As (
 select sde.st_point (10.01, 20.03, 4326) as pt from dual
)
select sde.st_geometry(sde.ST_X(p.pt), sde.ST_Y(p.pt), sde.ST_Z(p.pt), sde.ST_M(p.pt), sde.ST_SRID(p.pt)) as ptGeom 
 from point p;

Or you could use WKB.

My packages and objects have implementations of ST_StartPoint that return SDO_GEOMETRY objects. You could create a function that does the same thing but return a point geometry:

create function ST_Point2Geom(p_pt in sde.st_point)
returns sde.st_geometry deterministic
As
begin
  return case when p_pt is null 
              then null 
              when sde.ST_GeometryType(p_pt) <> 'ST_POINT'
              then p_pt 
              else sde.st_geometry(sde.ST_X(p_pt), sde.ST_Y(p_pt),
                                   sde.ST_Z(p_pt), sde.ST_M(p_pt),
                                   sde.ST_SRID(p_pt)) 
          end;
end st_point2geom;
/
show errors

select st_point2Geom(sde.st_startPoint(myGeom)) from dual;