Then again I have not read this book and do not know Forth.
then your opinion is invalid
>>1
older computers will use the forth shell as a bootloader. one of them is called openfirmware, but this is basically abondoned nowadays for shitty firmware.
so if you have to program or interact with these older computers than you will need to know forth to do more than running a simple command like boot.
besides that forth has not been used that often. i dont think forth is obsolete and prefer it to disasterous programming languages, but in the view of most people yes it is obsolete and outdated.
you will gain knowledge of how to write programs in smarter or better ways that are small and efficient to maintain compared to ungodly big programs that just keep growing and growing after time and become difficult to maintain.