I think what @Sam Lin is trying to say (and you should trust him, he's pre-med), is that your regular skillet really isn't optimal for what you're trying to do. Pre-heating it is better than cold, but it's still kind of half pregnant.
Using a regular skillet on a stove top to get a sear is using the surface of the skillet to give it the sear, as there is no heat source from anywhere else. In that case, skillet's are optimal. But, you're actually using the Ooni much more like a salamander in that you're using the ambient heat in the Ooni as your heat source. So, the cooking surface you're using really doesn't make a difference since the majority of the sear isn't coming from it anyway, it's coming from the ambient heat of the Ooni.
Now, if whatever surface you're using is cold, it will actually take away from some of the ambient heat of the Ooni and work against you. Additionally, a normal skillet with sides will actually work to block some of the ambient heat source from the Ooni too. So, using a cold regular skillet (which you did the first round) actually workrd against you double time...cold taking away from ambient heat and sides blocking ambient heat. Just pre-heating that will help, but it will still be working against you as the sides will block heat (which you can see is pretty clearly happening in your first pic).
That said, ideally, you should use something flat (no sides to block heat) and something pre-heated (so the cold doesn't absorb ambient heat). Whatever surface that is (iron, steal, etc) doesn't really make a shit. Just be sure it's flat (or mostly flat) and it's preheated and you should get your optimal sear.
Or maybe I'm reading it wrong, in which case @Sam Lin can clarify and delete / ban me and shit.
BTW...happy new year and congrats on getting the Ooni.