Sorry to say, but the chapter was clearly framing it as hope-fueled resurrection.
Ironside called his death and then later confirmed that he was definitely sure he was gone. He failed to kill him the first time, so we can trust he made sure he properly did the deed this time.
Nasiens and Anna clearly believed he had died. Nasiens used the potion, but being the Doogie Howser that he is, in his heart of hearts, he knew Percy was gone. Which is why, after Anna did the mouth-to-mouth transfer and noted the lack of swallowing, Nasiens finally called Percy's death.
This brings us to Ol' Donny. The only one still ignorant of what was going on. The chapter juxtaposed Donny's belief in Percy with the manifestation of the Golem in a single page. There was no struggle to manifest with the other two, there was no hint of this with them. It was all on Donny. And even after, the series once again reiterates using both Nasiens or Anna and Ironside as mouthpieces to say that, yeah, the kid was dead.
And then that single spark of hope was all that was needed for Nasiens and Anna to regain the hope they initially lost, literally using their bodies to protect that small inkling. And now it's here, and only here, where I'll say the potion came into play and not anytime before. When Donny finally arrived at the scene, there is a slight twitch coming from Percival, indicating that he's now capable of movement. If the potion was indeed still in his mouth, that's the time for him to swallow and increase his vitality to awaken him before he was fully healed by his own Golems.