Agree with Jack Trumbull competence and pricing, I had some work done by him on my Colnago Super. Excellent work (stay replacement) and very reasonable price!

I'm confused. Threading for the headset is cut into the steerer, so if they were ground off the measurement in that area would be ~24mm as mentioned by bulgie

You've shown us that the steerer tube diameter below the threads is 25.1mm and 25.0mm where the threads were allegedly located. The difference most likely attributed to the chrome finish on the lower portion, which would also explain the level of corrosion on the upper portion.

Maybe I'm missing something, but from the evidence given there's no way that the threads were ground off from that steerer tube.

I don’t think you’re missing anything. Here we are on a niche forum on the internet and none of this really matters but the numbers simply don’t add up.

Well I didn't want to mention your incorrect calipers technique, but now that yo've brought it up...
You are using the calipers wrong, and they appear to be plastic. I can easily make a plastic caliper read 25.0 mm or less on a truly 1" tube, just from the flex in the plastic, especially when measuring as far as possible away from the backbone of the caliper, the worst place to measure. And on the thin knife-edges, the most flexible part.

This pic you shared pretty much proves your caliper and/or technique are wrong, because you're getting 25.1 on the part of the steerer that is clearly 25.4 — or a little over that, because of whatever material is on top of the steerer. Looks like plating or rust, but it's not nothin'. That's not the part that supposedly got ground down, so it's 1.000 inch plus whatever's coating it. If you get a smaller number, adjust your technique (and get better calipers).

https://cimg2.ibsrv.net/gimg/bikefor...dc7b4e1658.jpg

Shame about the threads being cut off; it would calm things down were they found to be 25x1.