Is there a motor designed to be constantly overloaded?
Engineers use "overload" to indicate that that level is not intended for continuous use.

I mean, if a product is rated for say 900 W but a continuous overload at 1000 W is allowed, then why not simply call it 1000 W. As it can handle 1000 W continuously, the 900 W level loses its meaning.

What would make sense is 900 W continuous allowed load and an allowed overload of 1000 W for 10 seconds every 200 seconds. Note the time restriction. It indicates that you cannot have this condition continuously, only for a limited time.