Question

why does a hot liquid remains hot for long time in a thermal flask​

Answer 1

Yes, you are right, because the air is an insulator or poor conductor of heat. This is the only simple reason that trapped air in double glass will not allow the heat to loose and this is the reason that a liquid remain hot or cold for a long time inside a thermos.

Answer 2

because the air is an insulator or poor conductor of heat. This is the only simple reason that trapped air in double glass will not allow the heat to loose and this is the reason that a liquid remain hot or cold for a long time inside a thermos.