I don't think there's any evidence that the numeral 7 was derived from Zayin. The Indian numeral is clearly recognizable (it looks like an upside-down J), and was drawn with sharper corners by the Arabs, or even converted to a wedge shape.

The earliest Bhrami script examples we have are from 250BCE, about or even before the beginning of Late Hebrew usage, so would probably have diverged earlier. The best that could be said for this diagram is that it leaves out quite a lot of steps, if there is indeed a shared history.