does 실화임 and 설마 has similar meaning??

See a translation

No, not at all.
It's a completely different word.

*실화임 = it is true story (real story)
Usually it is used when we can't believe something.
So we are amazed by the incredible fact and use it as a form of question.

* 설마 = No way! / Are you sure?
In Korean, '설마' is an adverb, but I can't find an English word that matches this perfectly.
Instead, I'll show you an example sentence, so you guess what it means.

ex 1)
A : 내 동생은 늘 라면을 5개나 먹고도 배가 안 부르다고 말해.
B : 설마! 그거 실화임?

ex 2)
A : 내 남자친구가 나한테 날씬하다고 했어.
B : 실화임? 너 80kg 잖아. 설마 너 그 말을 믿어? 그냥 놀리는 거야.