It often is. Newer LED flashlights that run off a single alkaline cell always have a boost converter. Electronics that run off 3 cells often have a buck converter down to 3.3V or a lower voltage, which accomplishes the same thing, often a little more efficiently.
Boost converters from single cell alkaline batteries up to useful voltages are usually quite inefficient.
If you see something powered by two cells, it might not have a voltage converter because a lot of circuits can operate off that voltage directly.
If you use this sleeve inside a device with a built in voltage converter, you waste some energy, and your device gets confused about remaining battery life.
Boost converters from single cell alkaline batteries up to useful voltages are usually quite inefficient.
If you see something powered by two cells, it might not have a voltage converter because a lot of circuits can operate off that voltage directly.
If you use this sleeve inside a device with a built in voltage converter, you waste some energy, and your device gets confused about remaining battery life.