Ancient Greek in Mozart’s Kyrie?

I’m no expert in any of the things you’d have to understand in order to fully get to grips with this, but I wonder whether any experts have ever commented on this. (I couldn’t find anything through a perfunctory web search.)

The thing is: I always thought that the church version of “eleison” would be pronounced in thee syllables (“eh-lay-son”). But the rhythm of the Kyrie Eleison in Mozart’s Requiem is definitely more suited to four syllables (eh-leh-eh-son”) which would be more like the Ancient Greek version of the word for “take pity” then the Koine Greek. Did Mozart know Ancient Greek? Or would the “eleison” in his day and age be pronounced “e-le-i-son”?