Something isn't quite right about a 30mm saddle height difference between bikes. How could that be for a top rider?
What is similar is a saddle tip to handlebar distance of 580mm between bikes. This is a critical dimension for reach.

To answer your question....yes...a rider can ride multiple frames. For example when this rider turns 70, he maybe on a 58.
Same guy with same reach but lack of flexibility. On a 58 his saddle tip to handlebar distance will still be the same ~ 580mm with normal size stem
...but his entire cockpit will be rotated up and his handlebars will be level if not slightly above the saddle versus well below. And this is what an older rider of his dimensions would prefer without the flexibility or core strength of a top pro or amateur.


So yes, a rider can size down on a frame which will slightly reduce horizontal reach (top tube) but increase drop which allows the same reach from saddle tip to handlebar center on a smaller bike. This is what many pros do...they ride 1-2 frame sizes smaller than common riders because of their superior flexibility, core strength and reduced body mass.

FWIW, frame size is also largely a preference, even among top riders. For example, Lance who is virtually the same size as Gilbert rode a 58 cm Madone throughout his career with 580mm top tube and 130mm stem and only 75mm of drop...if that. This fact is what casts a dim shadow on the whole concept of a 'pro fit'. LA's preference to satisfy this reach which was the same as Gilbert...~575mm saddle tip to handlebar center...was achieved with more horizontal reach and less drop. Btw, this type of fit I believe is vastly better for the average rider than the trend of smaller frames with shorter top tube and more drop to achieve the same reach for a given rider size....because the reality is, more drop requires more flexibility and core strength which an average rider lacks.