日常生活有趣事一問(物理現象)

Before the lid is put onto the cup of boiling water, the steam evolves from the water pushes part of the air out from the cup.

When the lid is put in place, because the lid is at room temperature, the steam in contact with the cool lid condenses back into liquid water. Since liquid water occupies less space than steam, as a result, the air pressure inside the cup is a liitle bit lower than atmospheric pressure, the soft plastic lid thus is pressed inward by the excess pressure from outside.

although the water is hot ,it is not being boiled.That means the water is losing energy to the surrounding.More and more steam will change from gas to lquid with time.Steam which attach to the cup surface will condense into water again which occupy less space. So the lid will go inward.
Your assumption is true only if the water is being boiled.