Is this lint buildup in my tumble dryer really lint?
It is a sound absorber for the air inlet. It muffles the sound of the air being sucked into the dryer.
 Check the vent hood, where your dryer vents to the outside, for lint buildup. If everything is clear and your thermal protection till goes out frequently, you ought to have it checked out. It appears that the sound absorbers are deteriorated or at least loose and could be restricting flow. good luck