Well, yeah, the whole point of dehumanizing other people is to switch off empathy for them, which is why that's pretty much a core Fascist practice.

My point being twofold:

• The massive middle floats around and can be made to feel more empathy for others, or less empathy for others, including through tricks like dehumanization of people and on the opposite end things like good Theatre and Film to bring close to people the experience of others whom they never met (which, given that it's fantasy but with a real emotional backing - thanks to modern Acting techniques - can also be abused to leverage people's empathy to grow sympathy for a specific group even when the members of it mostly don't deserve it)
• There are people, at both ends of the scale who are pretty close to immutable on this, on one end those who can't empathize at all and never will because they simply don't have the brain wired for that (such as Psychopaths), and on the opposite end those who can't switch off their empathy and can at most temporarily suppress it.

More specifically to your point, this also reflects in how easy it is to convince people of the inhumanity of others - those with little or no empathy will easily believe it, those in the middle can be easy to sway with a concerting campaign of misportraying a group of people (which is what we see in for example places like Fox News when it come to immigrants, but also the New York Times when it comes to Palestinians) whilst those with very high Empathy will be very very hard to convince of that, and even if indeed everybody can somehow be made to feel nothing for a select group of others, the high empathy types would require a lot more work and done a lot more direct than the rest of people, and that's beyond what modern mass-media and social media can achieve by themselves - the system is setup to brainwash the bulk of people, as turning those is enough to make happen whatever the puppeteers want to see happen.

Your point is entirely correct and appropriate for the bulk of people but isn't correct for everybody - you're jumping to conclusions about everybody by observing the norm and, worse, in a society that currently is pretty much anti-Empathy: I mean, if even gender in humans is a lot more fluid than looking at the norm would lead us to believe, why do you think that it's possible to make any statement about Empathy that applies to everybody?!

This is how its possible for both you and the other poster to be correct.

That said, I agree with you that everybody should just in case not assume themselves immune to modern manipulation and thus should be eternally on alert for attempts by others to make their minds against "outsiders" (and similarly, to make their minds favor "insiders", such as the phenomenon of "copaganda").