NEJ, you are of course techinically correct that "tithing" implies giving a tenth of ones income on a regular basis.
In my experience here in the US, people often use the term to describe any amount of money which is demanded/expected on a regular basis by a church or other religious group, regardless of the exact amount or portion of an individuals income.
People are often less than precise with their language. Depending on the context, I tend to insist on precision in speaking and writing. But I also know that the WTBTS has a long history of playing word games in dishonest and weasely ways.
For example, there was a recent change regarding the "belief" of what happens when an anointed one dies and goes to heaven. While the GB did not explicitly refer to this "New Light" as the Rapture, it is for all intents-and-purposes what JWs now believe.
And so, while the New Donation Arrangement is not technically "tithing," the only thing missing is that they haven't demanded a precise percentage of the sheep's income.
There's a rumor in the wind that the WT leaders will soon direct the elders to inspect the publishers tax returns. This may or may not happen. But let's just imagine for a moment that the GB orders all JWs to contribute 9% of their income each month. That's not 10% so it's technically not tithing.
But what would you call that?