I ask specifically in the context mentioned below:
... It’s not just that you can’t get away with apostatizing from Islam: that if you’re a Jew you can’t convert to Christianity. If you want to exit Judaism, the only way you can do it is by converting to Islam ... On the Jewish side, I’m thinking of Maimonides in the Mishneh Torah where he talks about Rabbanites and Karaites. There’s a fierce bit where he says Karaites are apostates from Judaism and should be killed. Then he says, “I’m not talking about the descendents of those Karaites — it’s not their fault, we get along fine with them — but the people who actually apostatized from Judaism, they should be killed.” ...
from this article.
As I don't know jewish tradition, scripture and history, I don't really understand the point the scholar is making about "Jews can only exit by converting to Islam, and not Christianity". Can somebody clarify and clear my confusion?