Why would Fotudeng, a Kuchan, know any Jie?

What little Jie survives through his writings isn't enough to indicate that he himself knew Jie. He may simply have known a phrase that was told to him. That said, bilingualism wasn't really uncommon, especially for a missionary. It's quite possible that he did speak some, though of course we don't know one way or another.

You didn't actually ask about the language itself, but since we're on the topic, there's good support that it belongs to the Yeniseian family (Pulleyblank), and that in turn may well turn out to be Sinotibetan in origin. Assuming that was the case, it wouldn't have been too foreign to a Buddhist monk in the 4th century anyway. Even if not at all mutually intelligible with anything Fotudeng would have otherwise spoken, it would likely have been more approachable than, say, Arabic.

and did this indicate a failed effort by Shi Le to Jie-ize the state, akin to Manchu efforts during the Qing to make China bilingual?

China's always been multilingual, so I'm not really clear on what you mean on that point. However I don't think there's really much evidence to support the notion that they were trying to "Jie-ize" as you put it.