It is only endmass in the following sense:
1) The cueball is deflected in the transverse direction according to the mass of the cue that's put into motion in the opposite direction (momentum). If the end of a cue was perfectly rigid, but somehow lacked any effective endmass, no squirt would take place.
2) Stiffness, but in a sort of trivial way, is necessary for there to be any endmass at all (see Dr. Dave's reference to a wet noodle earlier). However, once you get to a stiffness level represented by a wooden rod, the differences in stiffness you're going to encounter amongst cues isn't likely going to make any significant difference in squirt. That's because:
2.1) Given the shape the end of the cue takes on as it bends, it's the mass nearest the tip that contributes the most to the momentum of the endmass.
2.2) Different stiffnesses primarily affect the shape of the bend further down the cue away from the tip. Because of that location, the additional mass doesn't contribute much to the cue's sideways momentum.
I once thought that stiffness was an integral part of endmass (it is in the trivial sense noted above), but repeated assertions by Patrick Johnson and Dr. Dave eventually drove home the "proper" view of it.
Now you can go.
Jim