They have only built one half of the steel structure, the other half of the volcano is completely open so plenty of time to put the slide pieces in before any remaining structure is built around. However I am not 100% convinced the current open side of the volcano is going to be full enclosed like the wave pool side. In the concept video from what I could make out there appears to be a few openings towards the back of the volcano giving the impression it is partially open, this would make sense with the amount of different slides potentially coming out of it. You will see below all of the open areas.
WildGravity Travel's latest pictures are in the link in the tweet below, take a look, some interesting images leaving me speculate quite a bit