They must be confident that the customer base will come back to them after such a long period with no service at all. They must also see no need to bring any kind of replacement in. Seems like they're handing the whole summer trade to Irish Ferries.
Its not that easy to secure any sort of cover given virtually everything that may be suitable for Fishguard is likely occupied at the moment. So, while as you say traffic is currently being handed to Irish Ferries, this is far from an ideal situation for Stena.
The current plan for STENA EUROPEs repairs does not involve dry docking.