Of course it's supposed to be the Executor. It was coloured blue in 1980 and again in 1997. That was their whole intention all along. You also have to remember that the executor model was more grey and it was coloured blue in post (check out the SE shot of the executor during the bespin escape where they forgot to reapply the colouring and it's almost white). It makes perfect sense to me that it would be the Executor. the stardestroyers are all small in the first shot then this huge tower comes into shot, then we see this large shadow covering the standard stardestroyer. Its all building up scale from shot to shot until the final reveal. The large tower, the shadow eclipsing the stardestroyer, the belly then the reveal. Without that first large tower there is no sense that it is a large ship causing the shadow until we see the belly. It could have just been anything. That is what an establishing shot is for.

If it was meant to be the Executor in 1980, then that is how it will be in this version.