I think it's a fairly safe bet that Merlin will work towards the assumption that they can reopen on 4th July, as that's what government advice indicates. Of course it could change, be moved back, but as others have said Merlin won't get specific confirmation that they'll be able to open on such-and-such a date, like any other industry (or schools for that matter - the first they heard about the 1st June target was Boris' national announcement).
So, to wait for such confirmation would be foolish - makes far more business sense to assume the parks will be able to open 4th July, and prepare for that, and then delay if necessary.