Even though Zhou Tian couldn't use most of his ability at that moment due to the environment of the trapped situation, the quality of the Equipment on him remained unchanged. In other words, even if the Equipment on Zhou Tian at that time couldn't exert its original ability, the iron-made items wouldn't turn into cloth just because they lost their ability.
Given the situation, unless Zhou Tian did nothing, even if he used his clothes as a Parachute, he wouldn't need to worry about his clothes not being able to withstand the Gravity during the descent process.
In fact, Zhou Tian actually did that at the time. As he kept falling, he thought about what would happen if he continued like that. Without hesitation, Zhou Tian used his clothes as a Parachute, using his Ability to control his descent speed.
