You Can't Violate a Policy if There is No Policy
If SW's policy on seat saving is that "there is no policy," then officially you are neither permitted nor prohibited from doing it. Take that any way you want.
When you have an empty seat next to you, you can ask the those boarding behind you to leave it empty. Say something like "Yes, it's saved," or something polite like "If you don't mind, I'm waiting for someone," or something rude like "Don't even think of sitting here." Or maybe do little tricks like not making eye contact, or leaving you laptop or jacket on the seat next to you, or wearing a surgical mask. Or you can sit in silence hoping the following passengers will read your mind. If you want. It's your choice.
If you are the one boarding later and see a seat you want, you may just take it. You don't have to ask. Any empty seat. Or you could ask politely if it's saved and if the reply is "Yes" then you find another seat. Or you still can take the seat anyway. If you want. It's your choice.
Are you type A or Type B? entitled or cooperative? courteous or pushy? Your personality type, not a vague airline policy, will determine what you do. It's your choice.