According to Chapter 921, the residents of Wano consider Kaido as Wisdom King, who protects Wano. The Fire Festival is the day for the Shogun and his subordinates to go to Onigashima to “pay their respects” to Wisdom King (aka Kaido). “Wisdom King” is why Fire Festival exists in the first place.
There are 5 Wisdom Kings in Vajrayana Buddhism. If any story uses Wisdom King as a reference, it usually refers to the Immovable Wisdom King (Acala).
And the design of prime Hyo is based on it.
Luffy’s Gear 4 Boundman (and also Enel’s 200,000,000 Volt Amaru form) are based on another guardian god called Nio. Nio are two wrathful and muscular guardians of the Buddha standing today at the entrance of many Buddhist temples in East Asian Buddhism in the form of frightening wrestler-like statues. They have a very similar design to Acala.
Therefore, Gear 4 Luffy reminds grandpa Hyo of Wisdom King. This plot probably foreshadows Luffy is going to be the true guardian god of Wano instead of Kaido after the war.