What the hell has 'Genisys' got to do with this? The reasons that failed have nothing to do with it being a retcon and everything to do with poor script, direction and casting.
'Terminator 2' effectively retconned some of 'The Terminator'. Are we also going to say retcon = success because of that, too? A lot of people also seemed to like how the recent 'X-Men' film has also retconned a lot of that history, too - and that was successful. Silliness...
The reason for this undoubtedly has everything to do with Ridley Scott being a producer on Blomkamp's enterprise. He won't be able to do an effective job of that if he's simultaneously so immersed in helming the 'Prometheus' sequel. To say that slight delay therefore must mean the other film will no longer happen, has no basis in reality. It's been green-lit - the budget has been authorised. It's simply a matter of timing, no more, no less. Hardly a first in Hollywood.
Also, look at the quote: "Alien is kinda holding/pending 'Prometheus 2'."
That means P2's schedule is responsible for altering A5's. It's nothing to do with whether or not there is/isn't a retcon or whether it would/wouldn't work. Assuming a retcon features, any potential problems Fox might have with that would have already been dealt with.
It's just Scott having his fingers in too many pies and needing more time to commit. Possibly also some story elements he's told NB he'd like to have crossed over, but which need revealing in P2, first. But almost certainly finding time for obligatory producer duties.