Even if it were conceded that such a thing is called composition, his statement:

“Every composite depends on something other than itself”

includes things combined by someone, such as physical compounds made from individual constituents—foods, medicines, drinks, and the like. It also includes things whose parts can be separated, such as human beings, animals, and plants; things whose different sides can be distinguished from one another; and things possessing inseparable attributes. This last meaning is what he intends here.

He may therefore be told:

“In that case, what is meant is that anything possessing an inseparable attribute must have that attribute in order to exist.”

This is true.

Even granting that you call this composition, it is not impossible for a necessary being. Rather, it is necessarily true of it, and the contrary is impossible.

Your statement:

“A composite depends on something other than itself”

means that something possessing an inseparable attribute cannot exist without that attribute, but you have called it a composite,