Yes, you're right, it is the isostatic unloading that they're referring to - but in regards to the 'sucking', they seem to mean that the semi-localised unloaded of the top of crust (via rain, river incision) creates a zone of lower lithostatic pressure, and so that area ends up getting pushed up to maintain isostatic equilibrium. Not truly a 'sucking', but as an analogy, I think it's OK. It's more easily seen as part of the critical wedge angle for fold and thrust belts.