I think that was actually Gojo. I think he was called a โghostโ because heโs Satoru Gojo and not โThe Strongest." The Strongest died to pave the way for the birth of Satoru Gojo (I'm speaking strictly in terms of personality).
In Esoteric Buddhism, it is said, "Aspiring to attain enlightenment means to make a wholehearted effort to uncover one's originally enlightened mind." It requires the development of human consciousness to ultimately unveil the 'true self', which is identified as the enlightened mind. The awakening experience is considered to be as powerful and shocking as a flash of lightning.
In the Mandala of the Two Realms of Esoteric Buddhism, Vairocana appears in the center of both the Diamond Realm and the Womb Realm. In the former, Vairocana appears as the Body of Wisdom, symbolized by Vajra. The vajra stands for the power of illumination, for penetrating insight that breaks through the darkness of ignorance and actualization. It is a symbol of the indestructible and irresistible truth. In the latter, Vairocana appears as the Body of Principle and is represented by a lotus, which stands for principle yet unrealized, compassion, potentiality, growth, and creativity. Both aspects of Vairocana are inseparably related. All these sound pretty familiar.
Gojo will return and open his domain with the completed hand seal. According to Esoteric Buddhism, the left hand represents sentient beings and the right hand is the Buddha, which thus symbolizes the two-way response of the Buddha and sentient beings. When you entwine both hands, you become one with Buddha. Gojo will attain enlightenment to fully realize his own sense of self, with Sukuna being the driving force behind that.
Changing barrier conditions, shrinking barrier size, and 0.2s DE were all precedents to show his potential. His DE with the completed hand seal will definitely get an upgrade, perhaps something akin to being divine.
Lastly, Vairocana is said to be the universal Buddha and consists of the six elements from which other Buddhas originated. He's the source of enlightenment and is referred to as a teacher who furthered Buddhism; without him, there would be no Buddhism and no path to enlightenment. This also parallels Gojo's decision to become a teacher or mentor and guide the youth. After attaining Nirvana, he'll guide his students to the same path.