Interestingly, if you accept that code can be copyrighted, and that only things that are expressions (speech) are eligible for copyright ("A copyrighted work must be an original work of authorship which is fixed in a tangible medium of expression"), then code == speech.
So, by compelling Apple to code something that doesn't exist, the government would indeed actually be compelling speech.
Court processes involve compelled speech all the time. Heck, compelling witness testimony, which is one of the most well-established parts of the court process, is nothing but compelling speech.
So, I'm not sure what the value is of a clever argument that compelling Apple to comply with the order here is "compelling speech" is supposed to be (likewise, the upthread one about NSL canaries.)
Fair points that I concede. I'll note I also forgot the nuance mentioned by morsch (that a canary is compelling a lie, which is potentially different).
On a separate note though, I've always thought it would be interesting to see a member of Congress be issued a NSL and then have them read it on the floor of the House/Senate (since they have parliamentary immunity for anything they say on the floor of the House or Senate).
Compelling speech is a thing that happens under certain circumstances. E.g. subpoenas, witnesses in court. Though those are clearly different from a canary, not least because producing such a canary would not just be compelling speech, but compelling a lie.
So, by compelling Apple to code something that doesn't exist, the government would indeed actually be compelling speech.