plainly stated that fork() alters the ordinary sequential execution of statements
No. It doesn't state that. It emphasizes that both before and after the fork() there is no difference except for fork's return value.
The only alteration is: after the fork(), there are two identical copies of those instructions. If there were is no if around the fork() (or you save the return value to a variable and evaluate it afterwards), you would not be able to distinguish between them. The "ordinary sequential execution" is not altered.
