Lots of factors could be in play here. What you're saying MJ could be completely accurate, though there's also this today, which is Fox buying another sci-fi novel and talking to Blomkamp about writing/directing:
http://deadline.com/2015/11/the-gone-wo ... 201618007/
If that pans out, then clearly Fox was behind Blomkamp's film (and who knows, still might be), and wanted to work with him. Landing him for another project is likely an olive branch for what went down on ALIEN 5, to say nothing of the fact that Blomkamp's planned HALO movie went south years ago.
What it feels like to me is that Fox -- obviously -- holds Ridley Scott in a far higher place than Blomkamp, and Scott WANTED to do ALIEN. If he wasn't that interested or only wanted to produce, my gut tells me they wouldn't be making ALIEN CONVENANT and
would be making Blomkamp's picture. Doing a sequel to PROMETHEUS could've gone either way -- it did well enough, but most fans' reactions were mixed -- so Fox likely was going to leave it going forward up to Ridley in terms of what they were going to do. (That would also make sense since it's taken longer than expected to crank out, and that this is being positioned as an ALIEN film, not a PROMETHEUS sequel).
Since Fox is going to do what Scott wants, and since he's serious about ALIEN CONVENANT and apparently another film behind it, then that's the direction they took -- and I fully agree with it. Like you said, he's coming off THE MARTIAN and he's been firmly set up at Fox now for years. Meanwhile, Blomkamp's output has been on a steady decline and as much as I want to see his ALIENS film, Sigourney is going to be elderly by the time they get to it. Hopefully it'll get made, if not reconfigured conceptually down the line -- assuming ALIEN CONVENANT does well enough.
That Fox still wants to work with him tells me they weren't down on him because of ELYSIUM and CHAPPIE, but that it was Ridley calling the shots on where these projects ended up. In the meantime, Blomkamp can prove himself to the studio with this new project, so there's at least that.