It was more specific in the novel tbf. Here's the exact scene there
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As if someone were teaching him. His heart raced. Emotions swirled— excitement, anticipation, or perhaps anxiety and confusion. Cha Eui-jae slowly opened his mouth.
"You…"
It felt like something was stuck in his throat, making it hard to speak. He wasn't sure. Even though he thought it was impossible, he still harbored a faint hope, a conflict of feelings.
A boy dying from poison, Lee Sa-young who used poison. A boy whose records were erased by Prometheus or some other hand, Lee Sa-young who escaped after being a test subject of Prometheus. A boy who tried to smile with narrowed eyes, Lee Sa-young who smiled with narrowed eyes.
'It can't be.'
Cha Eui-jae looked at Lee Sa-young with confused eyes. At that moment, Lee Sa-young smiled. His eyes narrowed, shining like a child's, waiting for the moment to open his prepared gift.

now of course i love that but how tf did he piece that together, that was a crazy pull.
and im actually impressed he went to that conclusion immediately instead of edging us and dragging it out so good job Euijae!!!