All the commands need to be executed. There are some exceptions, but if this is the case it is mentioned specifically by the book.
If you take a look at the above code block you'll see that the cp and sed commands are only executed in a specific case (if the output of the uname -m command equals x86_64).
BTW: If you're level of expertise isn't high enough to be able to understand what a relative simple command block like the above does, you might be in for a rough ride. I can only point you to what is being said in the LFS documentation: vi. Prerequisites
Hope this helps.