Holy crap, I have never ridden a bike more than 7kg's. Well, not a roadie anyhow.
My commuter is 10kg I guess, its a Merida T3 and wife also quite likes it.
The twitterer definitely needs a new bike... or even a decent old one. I just went out out and weighed my wife's 1980ish Cyclops Fiesta (full mudguards, chain guard, kickstand, steel carrier, padlock and cable, steel rims etc.) and it comes in at 16.5kg. She is only 150cm, and neither heavy nor strong.
Even my own old BSA, with everything, including a 6v lead-acid battery for the (old technology) extra headlights, comes in at only 19kg.
Some of the older "girls" (=smaller ladies') frames are actually reasonably light, in both 26x1 3/8 and 27x1 1/4 wheel sizes. The 26" sizes are hard to get rims etc for, but most of the 27" can at a pinch be fitted with 700C wheels, especially if mudguards are omitted. So hard rubbish scavenging may be the go for anyone inclined to build a light weight small ladies' bike with very forgiving geometry.
That is one heavy bike. My bike (tandem/cargo bike conversion) is under 20Kg! Lightness being the main advantage of aluminium.