I fully realize there is no purpose to my existence, but that means I can do what the fuck I want and not worry about wasting my time.

He can't prove there's no purpose, that takes faith. Put a person in a group of people who say the earth is flat, that person will probably say the earth is flat. It's people like you who seem to very consciously sustain those groups. This particular group called nihilism has led to many depressions, suicides, and criminal behavior. He can't prove any ultimate lack of purpose (he probably doesn't even know why his soda cans are designed the way they are), and in the body of his post he actually shows that his behavior has been changed according to a cogent line of reasoning (which necessitates him having donned a purpose at some point, such as having a higher quality of life).

I just want to spend my time here with as little worry and as carefree as possible.

nothing of it matters in the end.

Yea it does matter though otherwise he wouldn't have changed his life. Also we don't even know if there is an end. Mattering is furthermore something that everyone already knows is ascribed subjectively, and he has stated himself what he thinks matters-- his happiness basically.