The coil is detachable from the bulb vessel but I guess there was a dead short because the ballast emitted a buzz before it went pop. But the guy also checked with a meter the coil there was some reading. I think even if the vessel lost vacuum it still shouldn't pop like that should it?
It should not, in such case the ballast should detect it and protect itself by shutting down.
Servicing with power ON: Usually the given serviced fixture could be switched OFF (remove a fuse, circuit breaker, connector,...), while keeping the rest on the circuit (or on the pole) burning, so you do not have to deal with live parts.
And with all decent installations the ballasts (and that include the ignitors) are designed to survive the missing lamp at least for few minutes, so allow the lamp to be replaced. But some ballasts (and/or ignitors) shut down into a "fail safe" mode and need the power to be cycled to reset them, so allow the new lamps to be started.
And with advanced computer systems, if well designed, it is even way easier to shut down the lights to be serviced - all I have seen allow at least some kind of manual "service mode", where you control the individual fixtures exactly in the way you need for the maintenance (shut down and let to cool those, where the bulb is to be replaced, restart those with replaced bulbs)...
