There is a lot of work required to pour the concrete base for the track bed, so there is a flat floor in the (currently round) tunnel. Plus all the utilities, cabling, lighting, drainage, etc etc. There must be a fair bit of grouting required too, I don't know when that is done.
I recall from the TYSSE that Youtube videos of the "actual" TBM breakthroughs surfaced well before any official photo-op breakthroughs happened. It's apparently commpn industry practice to seal up the end of the newly-finished tunnel end and stage a second 'breakthrough' for photo op purposes. Since the TBM crew will demobilise shortly, I bet lots of workers recorded the final breakthrough for their personal memories. There's not much ML can do to suppress the news that the TBM's are done....ML may have chosen to 'go with the flow' as word would leak out anyways.
Or - Maybe that's what the extra time in the schedule from now to late 2017 is for.....a never ending series of photo ops before the tunnel is handed back to the contractor to let work continue.