I got out my calipers --- the top tube, head tube, and seat tubes are 29mm (most likely 28.6), and the downtube is 31.5mm (most likely 31.8). It doesn't feel stiff to me, but since I broke 2 ti frames, my guess is each time I broke one my custom builder proceeded to build me bikes with thicker/heavier tubing, so it's quite possible that my ti touring bike is so stiff that OS tubing feels right. The seatpost is 26.8mm, so that means the tube is 1.2mm thick at the ends and thinner in the middle. Since the tubes used by Rivendell aren't disclosed, you'd have to ride it to see how stiff you think it is. I don't have a 1993 Bridgestone RB-1 to compare, but when I rode that one I wasn't nearly as strong or as abusive to bicycles as I am now. I was considerably lighter (113 pounds), and wasn't taking my road bikes off pavement, jumping ditches, etc. (That RB-1 died when a Volvo hit me)
Looking at this page:
https://www.veloduo.co.uk/blogs/news/steel-frame-tubing-fact-and-mythology, the typical steel frame seat tube has an outside diameter of 28.6 (which is pretty much what I measured). I know Grant is conservative about tubing (which is why I'm so confident about riding the Roadini off pavement), so a wall thickness of 1.2mm (being 0.4mm thicker than the 531 standard) wouldn't surprise me.