According to the second version of the Gemora, Rabi Yishmoel ben Elisha actually moved the light
He says that he is therefor chayov to bring a korbon chatos.
From the Gemora it seems that he moved it inadvertantly.
If so, there is no korbon.
A chatos is only brought if what was done, was done purposefully, but he either - a) forgot it was shabbos, or b) forgot that it was osur.
Otherwise there is no chatos.
Rebbi Yishmael Ben Elisha was so involved in his learning that he forgot that it was Shabbos. Your question -- that he was *Mis'asek* and should be Patur from a Korban -- would only apply if he did not intend to do the action that he did. Rebbi Yishmael, though, fully intended to tilt the lamp. He just did not realize that it was Shabbos.
All the best,
Yisrael Shaw