I am no newbie. It ain't no ordinary wobble. It was wobbling like crazy.
Wobble when coasting is common and normal within a range. It happens because the cassette spins on an axis which is itself spinning with the hub. The two axes, hub to cassette inner & cassette inner to cassette outer don't align perfectly causing the wobble.
Normal is a matter of degree, 1-2mm is fairly common, more so might indicate that the cassette body is partly unscrewed from the hub. You can do a quick check by removing the wheel and seeing if you can move the cassette with respect to the hub, but the only definitive check is to follow the procedure for replacing the cassette for your particular hub. This often involves removing the cassette, then the axle, then accessing an internal screw or locknut inside the hub with a 12mm hex key.