No, it wasn’t.

Trees (esp. those suitable for paper) either burn or fall over and rot, both of which release most of the carbon they used to store.

Fossil fuels used to be trees, and they were well sequestered. Paper would eventually become something similar when buried.

I assume any serious paper production company replants the tree, so you start with a tree and end with a tree and landfilled paper.

Given strommen's statement about landfilled paper turning into methane, doesn't that mean paper manufacturing is basically a carbon dioxide->methane conversion operation?

Yes, the possibility I described was for if they did not biodegrade significantly. More reading makes it sound like they degrade in weeks, with a few exceptions.