"Crash dieting", as commonly practiced, is to eat less until your weight goals have been reached, and then go back to "normal". IF is hardly that; instead, it's a controlled pattern of regularly, and for specific durations, reducing your caloric intake. Some IF practitioners use a 6:1 cycle, where they have one fast day a week, some 5:2, and yet others 25:5, as in TFA. Some just take a random fast day whenever they feel like it, even.

IF is much less about weight than it is the other, generally scientifically demonstrated benefits of reduced caloric intake — though it does tend to confer some benefit in weight reduction as well.