I think
@Phantasiastisch's point was more that at the start of season they would say about ERT being available after the summer holidays in Germany too, whereas now they're just mentioning it before.
It's clutching at straws, but could mean something.
It is clutching at straws really, because last year the park offered "1 x zahlen, 2x Spaß" from April until June, and you could use your second return visit at any time during these 3 months. The fact that they aren't offering the same offer this year could either indicate:
1. They have invested millions of euros in Rookburgh and are now struggling for money, which is also indicated by removing the hotel discount for Phantasialand club holders
2. Rookburgh could open in April, and they don't want people having 2 visits for the price of one in the opening months of a brand new attraction.
I'm contemplating to book a trip for the start of the season, as my hotel discount is still valid until July, however am unsure how likely it would be that Rookburgh would be ready for then. Obviously it is much more feasible that the new themed world will be ready for end of June time, similar to Klugheim, however there's still every possibility looking at progress so far that the land will be ready for the start of the season.
I'm visiting Hotel ling bao for Fantissima mid-march so I will take as many construction photos as I can visibly see then, and that should be a real indicator of when we are looking at an opening date, depending on how far along the construction is a couple of weeks before season opening.