There are several ways that indoors training and HIIT workouts assist in fat loss, as I assume that’s the weight loss that you are targeting. A quick point though, fat can go down while weight remains the same or increases. Muscle weighs more than fat for the same volume/size, and having more muscle can increase W/Kg still.
Anyway, doing the intense workouts themselves obviously burns calories, but there is also the excessive post oxygen consumption (EPOC) which is why you stay warm after a HIIT, as the body is still using oxygen and burning fuel at a rate higher than it would at rest. So the calorie burn is extended.
Another benefit for fat loss, coming back to muscle again, the more muscle mass you have the higher your resting metabolic rate, so the amount of calories you burn a day even at rest or sleeping will be higher.
A few things to bear in mind though, if you were riding for more duration outdoors in the summer, then you likely had a higher calorie burn. Let’s say a tough indoor session uses 800kcal an hour, but a light outdoor session probably still burns 500-600kcal per hour. Outdoors generally with have a higher overall energy usage due to the duration. Another factor is that HIIT generally gives us more of an appetite after the session. It’s important to refuel with carbs and protein straight away as this will prevent excessive food cravings later on in the day.
One final point, it is more difficult to be in a calorie deficit after HIIT. HIIT uses carbs, and this is not something we have a huge store of, and although we burn fat after through EPOC and resting metabolic rate, fat burn during is limited. A longer slower endurance ride will use more fat as a fuel source, which although needs some carb and energy intake during the session and the rest of the day, does require the same priority refuelling that HIIT does both to perform it and to recover from it.
It does come down to kcal in vs kcal out, but there are several factors that affect this input and output. So you can shift weight with shorter indoor HIIT, but you need to refuel straight after to reduce craving later, and take into account the overall intake.