I suppose one can never really account for every possible variation ...
The fact remains that EVERY such 'egress' light I've seen has been some manner of HID. Maybe we are differing in what is meant by an 'emergency' circuit.
For the egress lighting, it is only required that they provide light for 90 minutes after power failure. So, many fixtures have batteries within the housing.
Another variation is to have a circuit of ordinary fixtures that is also supplied by a battery / UPS bank. No flicker / restrike issue there. Why have such a system? I believe it arose because many buildings have multiple tenants with separate metering - while the outside lighting is controlled by a 'house' panel. When the remodel of ONE unit results in the addition of "egress" lighting, sometimes the easiest way is to simply add a battery back-up to the existing outside lighting circuit.
Look, we're not here to debate various code and engineering nuances - at least not in this thread
It's just that this well-meaning Handy-Andy got zapped. The idea is to remind the rest of us to make damn sure the circuit is dead, then work like it was hot anyways. Even with a dead circuit, those fixtures have capacitors that can zap you off your ladder; only the coroner will really care what it was, exactly, that killed you - the shock or the fall.
Other times those fixtures are wired to 2-pole circuits, and wires get mixed up. As a result, one of those feed wires might still be hot. Oops. So we check - then check again.