I think of bunch of it would depend on your history as well. If you did several years doing big hours then switched to fewer hours I would bet you would have greater success. I would be very surprised if gsteinb never did any big volume weeks. Wasn't mattm a randonneur before focusing on sprinting?
Another thing to consider is that highly-structured indoor training should carry far less chance of experiencing a setback due to crash or injury. So even if 8 hours is less effective training than 18 hours if you have much better consistency over the medium to long term you could end up further ahead.