Unsharpness due to camera shake increases with magnification. The "rule of thumb" seems to go back to the 1950's and seems to be based on a 1/100 edge blur being acceptable in a 10x8 inch print taken with a standard lens for the format of a 1 foot wide format. Half a century ago few people had anything other than a standard lens, zoom lenses hardly existed, and film was not good by modern standards. Modern equipment is much better and can be enlarged much more - and the 1/100 blur OK in a 10x8 print can easily become 1/15 of an inch from 16 MP viewed at 200% on a monitor. If your technique is reasonable you get sharp images from any camera in a 10x8 inch print. If you want to make a print after cropping a lot, or to view at 200% (as you can from 16 MP) your technique needs to be very good for camera shake not to show at large magnifications.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.