Those ArcPy functions utilize the
SgShape functions lower in the API stack. The ST_GEOMETRY object was built using some of those same functions, but doesn't expose them all (just the set that implements those required by Spatial Types and Functions implementation, which doesn't include LRS).
The LRS primitive list exposed at the ArcSDE API isn't large:
extern BOOL SDEAPI SE_shape_is_measured (const SE_SHAPE shape);
extern LONG SDEAPI SE_shape_find_along (SE_SHAPE shape,
extern LONG SDEAPI SE_shape_find_between (SE_SHAPE shape,
extern LONG SDEAPI SE_shape_interpolate_by_measures
The rest is implemented as part of ArcObjects.