There is nothing "obvious" about a refresh being imminent--that is someone's theory with no backstop as far as I can see.
Tesla also does not have "stock" they need to flush. Everything is built-to-order and batched with a just-in-time supply chain so the component inventory is kept lean.
If you look at the history of the S and X, they just make rolling changes so you end up with a better car than you thought you were getting. They do the with big changes like AP hardware (including the original rollout) and the refreshed nose, but also a bunch of little things along the way.
Finally, as lean was the company runs, I cannot see them trying to refresh two models at the same time, especially since the X is only three years old. I would imagine every spare cycle has been devoted to getting the Y out the door and then pivoting over to the pickup later this year.