“I am this” speaks for the embodied condition in which mind has localized itself by a powerful desire and propensity to be a formation of the Five Aggregates, knowing nothing else. In nirvana, on the other hand, the "I am this" (i.e., the Five Aggregates) and the world perceived by means of the senses, is surpassed. One directly beholds a field or plane that is utterly devoid of phenomena—yet it is substantially real, in fact, more real than any phenomenon. (In Zen it is described as thieves breaking into a house that is empty—the house is real but it is totally empty of things to steal.) Coming with this realization is also the realization of one’s independence from sensory dependence which is liberation from the wheel of samsara.
For one who has achieved such independence (i.e., nirvana), they fully understand the danger of being dependent on phenomenal arisings which are the result of mind as sensory consciousness (vijnana) determining itself objectively, as something sensory. This can be quite delusive because for sensory consciousness there is presupposed nothing besides sensory phenomena—even subtle thoughts. However, this is not even close to the apperception of the pure field of nirvana.
It should be underscored that phenomena, of which we are conscious, are never in nirvana. To look for nirvana in the sensory world around us through our sensory consciousness is like being on a vicious circle. No matter how much we refine our search, we are sensory bound, unable to see the escape (i.e., nirvana). It is only when consciousness transcends the sensory realm that it is set to apperceive nirvana.