Thanks for checking it out! I agree with the "early" wording . I'd clarify that if I could. It's just saying early, not "You did it wrong - you were too early!", which is confusing for sure.
Basically, when the (invisible) timer hits zero, you get attacked. On the dot. The vampires have a window before that where they expose their heart. Sometimes (usually) that window is not long enough to wait around and twitch-react to the heart popping up, so you have to aim to start drawing your weapon just before that window so that it's open when you get there. But not TOO early, or else your accuracy goes to heck.
I hope that makes sense. Cheers!