Yes, it seems likely.

I assume the author would have noted in his blog post if the Hebrew used in making the threats came across as non-native or non-fluent.

Machine translation is really good these days, but I still think there are some "tells" for a truly vernacular and local command of a language.

Because translation is so very hard?

Yes, for colloquial speech to be plausible to a native speaker.

I assume the strange English translations of some of the Hebrew messages are because the messages were badly translated to Hebrew. It certainly sounds like the kind of result you'd get out of Google Translate.