There seem to be multiple definitions.

I'm specifically talking about human trafficking the way it is described in the article: people either being moved against their will or tricked/forced into a situation where they are being held against their will (e.g. by taking their papers and forcing them to "pay off" their debt as sex workers). This meaning is called "Menschenhandel" (literally "trade of humans") in German.

Calling illegal migration "human trafficking" isn't particularly helpful though it's probably a valid use of the term (the analogy being drug trafficking, or the illegal transfer of people or objects across borders).

Sure about that? I thought the term refers to people who are brought against their will or by lie to another country.

Definition from google:

the illegal movement of people, typically for the purposes of forced labour or commercial sexual exploitation.

Smuggling immigrants across a border is still "the illegal movement of people" and counted in the trumped up "human trafficking" statistics that are bandied about.

So what? Even if the numbers are inflated it's a real problem. Forced prostitution and force labour are valid problems in their own right, even outside human trafficking.