Right, because the ESFP introverts in other contexts. ESFPs love to be the center of attention and thus they are often labeled 'entertainer' or 'perfomer.' However, when Fi comes forth, the fun and spontaneous ESFP becomes rigid and judgmental, even preachy, when provoked by signs of social injustice.
But if Mercury was an ISFP, then why doesn't every ISFP appear to be an ESFP on stage or desire to be an entertainer of some sort?
The MBTI is a weak theory, and giving it a Jungian spin (as has been done over the last 20 or more years) doesn't take care of any of its flaws. Or where it does, it only produces new flaws.
The ISFP person can type as many Enneagram types, often as 4w3, 6w7, or 9w8. These e-types are psychologically conflicted and contradictory. The 4 is a quiet, unassuming, and socially introverted type, while the 3-wing seeks social success or achievement. The 6 seeks to merge safely with and disappear within a social group or ideology, while the 7-wing is more flamboyant and socially risky. The 9 seeks merger with an individual and is very devoted to that person, while the 8-wing is territorially expansive and seeks adventure, feeling held back by relationships.
The ESFP person, on the other hand, doesn't type as many different and contradictory e-types. That mbti type statistically correlates with 7w8 and occasionally 8w7, and only rarely with other e-types.