That is a fun fact, and also explains why manhattanhenge only happens at sunset and not also sunrise. A true east-west sunrise/sunset occurs at the equinoxes (Spring and Fall)

It does happen at sunrise roughly 18 days before and after the winter solstice. (Dec 3 and Jan 10)

It does happen at sunrise, just on different dates, around the Winter solstice

I take it this was due to the magnetic North at the time perhaps?

It's because the roads are aligned with the axes of the island itself, and the island doesn't face north-south