I'm not convinced. I can't say what the real situation is, but it isn't that. You can't grind the threads off a steerer and then clamp a stem to the resulting uneven, random-sized surface. It would have to be way smaller than your caliper shows, to make all the thread go away — the minor diameter of the thread is around 1.5 mm smaller i.e. somewhere around 24 mm. That's how far you have to "grind" a steerer down to make the existing threads all go away.

Plus we can see in the pic that the original threads aren't gone, they're still there at the top.

Again I can't tell what all is going on here. I'd need to inspect the fork myself to say anything definitive.

I will say though, threading it to French is unsafe, for reasons. For one, the inside diameter is smaller on a French steerer. Taking a steerer down on the outside, while keeping the larger inside diameter, makes the wall thinner. Steerers do break sometimes and the results can be catastrophic as in dead. There are other reasons this is a bad idea but that big one should be enough.