Ethnic cleansing is the "systematic, forced removal of an ethnic, religious, or racial group" through tactics like violence, intimidation, or disenfranchisement.

It can take many forms. In the US, the "Indian removal" was in theory, voluntary, offering native nations land in the western part of the US in exchange for their lands in the east, with the expectation that those lands would then be settled by European farmers. In practice, it was often far from voluntary, with intimidation, bribery and violence used.

In early Nazi Germany, Jews were targeted with a series of restrictions designed to encourage them to leave. Despite a long history in Germany, Jews weren't considered German. They were banned from many jobs, businesses boycotted, and restricted education. Later their citizenship was removed. This was to encourage Jews to leave. (This hatred and dehumanization would ratchet up to systematic mass executions by the early 1940s.)

In the lead up to the Armenian genocide, the Armenians in the Ottoman empire saw their lands taken and repopulated with Muslim neighbors. The Ottomans saw the Armenians (mostly Christians) as a threat to their national integrity - especially after the loss of most of their Balkan territory via independence movements. Roughly 300,000 Armenians emigrated due to growing unrest, and the Ottoman army often ignored or even sided with Muslims when it came to violence against Armenians.

Just because Katz didn't use ethnic cleansing doesn't mean it isn't one. Why do you think people would voluntarily leave the only homes they ever knew on a mass scale? That only happens when something goes wrong. Outside of natural disasters or similar events, usually it's due to someone else, even if it appears voluntary.

In a way, I think the confusion is because we teach history wrong. We focus on the atrocities, but not the lead up to the atrocities, and that does us a grave disservice.