Gliding away silently is often a hybrid. London does own electric buses, but mostly it has hybrids (and lots of conventional diesel buses).
If you were on the bus its engine probably started a minute later, it's still an improvement, and I assume that environmentally not spewing fumes at bus stops helps a little with local air quality, but it's not usually an electric bus.
Considering that most of a city bus's use is at low speed, constantly braking and accelerating from stops, the hybrid should be a huge improvement environmentally; it's the perfect use-case for a hybrid powertrain (where the ICE just runs at constant speed and charges the batteries, known as a serial hybrid vehicle).
If you were on the bus its engine probably started a minute later, it's still an improvement, and I assume that environmentally not spewing fumes at bus stops helps a little with local air quality, but it's not usually an electric bus.