Calling the shadow an "ESFP" is shorthand. It mostly means that the INTJ, for reasons of some importance, finds that intuition and thinking DON'T provide answers. Maybe the environment doesn't need them, or whatever is going on repeatedly tells the INTJ that everything he's doing is failing. And he wants to not fail, so he resorts to feeling as the judgment function. And, more importantly, none of his imaginings are producing any pathways out of the mess, so he resorts to sensing to try and see the way forward. To actually have the shadow rise up and take precedence, the situation has to be on going and inescapable. Being emo for a bit or going on a binge once and a while isn't the shadow. It's a hint of the shadow, because the shadow's always there, but for the shadow to rise up and take over, that's something different. The shadow is when normal functioning isn't possible, and hasn't been possible for some extended time. It's when you have to seek a solution by running from your normal solution seeking measures, using the energy of distress to power whatever's left of your ability to function. It's an "unhealthy" ESFP because it's unpracticed and immature, but mostly because it's a driven response, and unnatural. The priorities are for escape, not for being normal. Your personality becomes a means rather than an end in itself.
Possibly over-emphasising stress and take-overs here. The shadow is always present inasmuch as the lower functions are always present. Teh Shadow<tm> is a state where the higher functions abdicate.