Very interesting indeed.
I would expect sub 20lb retrobikes not that difficult tbh, my oldest bike a 59cm late 50s Carlton 531 is 10kg/22lbs.
That includes heavy Olimpic platform pedals (+clips and straps), Shimano RX100 dual pivot brakes, std alloy bars/quill stem, water bottle cage, pump pegs and a long cage Golden Arrow derailleur.
The only real concession to lightness are the Stronglight 99 50/36 compact c/set (570g) and a pair of retro lightweight 36 hole clinchers (sub 1700g sans skewers)
I haven't really weighed my gitane GTI triple butted but it is pretty light even with the heavy 8 speed 105 STI's on it.
My most modern race bike (58cm principia Rex Ellipse) recently reshod with Shimano 6700/5700STI's comes in at 7.5kg including cages.
Concessions to lightness are lightish tubular CF wheels, FSA carbon c/set, CF bars & ti/ally chain. I actually made it heavier by swapping out the DA 9 speed shifters/cassette for 105sti's and Ultegra cassette.
1958 Carlton Super Python
1990's Gitane VITUS GTI Triple Butted
1996 Raleigh Titanium Special Products Division
2007 Specialized Globe Pro
2007 Specialized Globe Expert Ultegra 6700
KTM Strada 6000 Carbon Dura Ace 9000