Losing weight is a consequence of maintaining a net negative energy balance, i.e. metabolising more calories than you consume. It's no more or less than this physiological fundamental. The gear you use on your bike is of little to no relevance.
The total energy you metabolise each day is a combination of your basal daily metabolic output plus whatever extra calories you metabolise through exercising and other activities. If cycling then that's simply a function of the average power through the cranks (watts) multiplied by the duration you rode for (seconds) divided by an efficiency factor (approximately 0.21, since humans are not 100% efficient at converting fuel substrates into mechanical energy).
In order to metabolise more than you currently do, the trick is to ride harder for longer. However our ability to ride hard comes with some limitations on our ability to sustain the effort level for very long or to consistently repeat such effort, and so to increase the calories metabolised most people simply ride for longer durations.
As for calorie intake, that's all about what and how much you eat and drink. It's way easier to consume calories than it is to metabolise them, so keep that in mind when considering where the priority lies with weight management.
As a rule of thumb, think about training being mostly about improving your fitness (power output) while you eat in a manner to get lean.
With respect to what your bike shop said, there's not a lot of context to go on but my view is if you are cycling, enjoying it and not harming yourself or anyone else, then who cares what gear you choose to ride? Just have fun.