That comes from the Mid Term Development Plan (MTDP) which was submitted to the council in 2010.
Basically, Thorpe used to submit a MTDP outlining their vague plans to the council. It would give rough locations to rides, coasters and their heights. The council would then approve those plans, so when the park put in planning permission for new attractions, the process was much quicker. It was beneficial for both sides.
The 2010 MTDP outlined two coasters - one of a max height of 40m, one of a max height of 50m. These were vaguely planned for 2012 and 2015. The 40m one is of course Swarm. After Swarm, the park and Merlin changed plans pretty dramatically (4 year major investment cycle for example), and eventually decided to go for a non-coaster (Derren Brown's Ghost Train) for 2016.
Island A is the island behind Swarm and Stealth. It's long been earmarked for a development. However, the 2010 MTDP is of course very old; most of the plans were drawn up in 2009 and only foresaw the park vaguely up to 2016. So they basically mean nothing now, so nothing to read into.
The obvious question now is where is the most recent MTDP from 2016 onwards. At the moment, there isn't one. Thorpe put in an application back in 2017 which basically asked for them to not have to put planning applications in for basically everything that wasn't a coaster. That was met with reservations from the council and concerns from some locals, and a decision still hasn't been reached either way. I don't expect them to get approval for that though, for many reasons.
Tl;dr - that quote comes from a 10 year old document and means nothing now. Don't read into it.