It's actually a good advice. A class not designed to be extendable should be marked as final.
But 1999, that was peak "inheritance is how we fix everything" thinking.
About the only reason to not do this is if you are writing a library. Even then, it's better to make extensions available through things like closures (not really available in 1999).
Properly designing classes for inheritance takes proactive care and is, in my experience, almost never done unless the author has been forced to by external forces (through APIs or agreements with other developers).