Hydrogen embrittlement is a volume effect: it affects the volume of the material. Environmental embrittlement is a surface effect where molecules from the atmosphere surrounding the material under test are adsorbed onto the fresh crack surface. This is most clearly seen from fatigue measurements where the measured crack growth rates can be an order of magnitude higher in hydrogen than in air. That this effect is due to adsorption, which saturates when the crack surface is completely covered,  is understood from the weak dependence of the effect on the pressure of hydrogen.
Environmental embrittlement is also observed to reduce fracture toughness in fast fracture tests, but the severity is much reduced compared with the same effect in fatigue
Hydrogen embrittlement is the effect where a previously embrittled material has low fracture toughness whatever atmosphere it is tested in. Environmental embrittlement is the effect when the low fracture toughness is only observed when the testing happens in that atmosphere.