It’s been at least 13 years that the meaning has changed. Since the introduction of the 13 inch MacBook Pro.
In my experience with that whole line I really feel like only the 17" truly deserved the "pro" moniker since it got better options (processors, matte display) than the others.
https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Retina+Disp...