The placebo effect is amazingly strong.

I'm a big dude - 6'-3", 280 - and I can flex SLX like a rubber band. Same thing with Gara, a tubeset supposedly designed for touring and amateur racing. I could stand and sprint and practically shift the front chainring. Sold off both my vintage bikes because of this.

My Gunnar with modern Platinum OX, however, was solid. The ride had all the steel smoothness I'd been told about, but none of the rubbery flex. It flexed, of course, and I could stand next to the frame and push down on one pedal and see the BB moving. The amount it flexed, however, was just the right amount so I got the benefits without it throwing off my sense of stability or balance.

I'll bet most vintage riders raving about the feel are trim and small build, or super casual riders, and therefore never stress a frame much. That's why they can rave. Get a big strong dude like me on, (or a decently big man with a 50+lb. touring load), however, and classic tubing will show it's weakness.

Edit: when I was taking Yamaguchi's course, he upsized every single major tube on the bike to beefier, OS tubes, and drafted it with compact geometry. This was on a bike that would see credit card touring, maybe 25lbs. on the back. If you're not a gorilla like me, YMMV.