They are stating that they intend to force this question in court if they have to, but are going to wait 10 days before filing to give both sides an opportunity to solve it in a simpler way.

It is arbitrary, and it really is nothing more than a letter. But it sets expectations on what consequences may arise if they choose not to respond. It is now up to Wizard of the Coast to decide whether and how to respond.

* they could have a book going to the publisher (book or website) in a few days

* forces the issue, and demands action from WoTC; fairly common

* keeps the issue front and center, ensures it is visible for the rest of the community, which means they can act accordingly (instead of it quietly happening behind the scenes, followed by targeted Cease and Desists, settlements, etc.)

* only requires a response; Hasboro might be able to buy time, offer an olive branch, etc.