Inferno did not do anything until the very end of the battle. In the aftermath of summoning Raya Lucaria Soldiers, a spirit ash that you can find within the road's End Catacombs that calls forth three warriors, Inferno was seated and watched the fight like an anime boss, waiting for the fight to Elden Ring Runes end before he took on the rival. Inferno was hit once however, as their health was high, it was not an unfathomable blow. After they were defeated by the Raya Lucaria Soldiers were eliminated, after nearly killing Inferno Tree Sentinel by themselves, Inferno was able to clear the place with a swift sequence of spear thrusts. It was hilarious and thrilling to witness.
It wasn't clear to everyone that the video was actually a joke. Because they had written "Game is difficult" in the video people online were confused by the idea of Elden Ring isn't as hard as it really is. (It is!) A few were mocking Inferno for defeating the "beginner boss." Others offered their strategies to defeat The Tree Sentinel, most of which involved returning after having a little more experience. This is the most likely explanation of how these soldiers were capable of taking on the boss as well.
I love the cheesy enemies such as this, and FromSoftware appears to promote this with the ability of summoning both spirit-ashes as well as real-world players playing the game. I used a similar strategy when combating The Tree Sentinel, calling upon my ghostly wolves and receiving help from a magical-based warrior. We completely destroyed him, however, it wasn't like the Inferno method. People are becoming proficient in Elden Ring, killing enemies with fashion. On my side I'm not likely to buy Elden Ring Items ever make the credits.