What’s wrong with reading a book, writing poetry or a novel, exercising, playing with the
They have the wrong idea here. Most people who have nothing/nothing meaningful to do at work still have to be present and give the semblance of working. Or do bullshit tasks that nobody will ever check, but if you don't do them you risk losing your job and your income.
and going home to enjoy your hobbies fully rested?
Being bored out of my mind is extremely exhausting to me.
This. And even if you do have time to let your mind wander you can't actually "do" your hobbies at work. At least for me I can't do any personal programming projects while working or my job will own whatever I do.
I don't even plan to sell anything, I just want ownership of it so I can't even fill my time with that.
Easier said than done, but yes this is very true. It's way too easy to fuck up and touch the wrong account, or sync the wrong data, or otherwise contaminate your personal projects with work.