That may or may not be true for the Universe as a whole - I have no way of knowing and speculation is useless. But that energy cannot enter or leave a given subsystem of the Universe as a whole is certainly not true. Take the Earth within our solar system - energy both enters and leaves the earth. Take the thin skin of biological life upon the Earth. Energy both enters and leaves it. Take the tiny sub-system of humanity within the thin skin of organic life upon the Earth. Energy both enters and leaves.
Do you see where I am going with the anologies?