Scott also has The Cartel (where his involvement was announced in July 2015) and The Prisoner (where his involvement was announced last January). So it could be that he does Wraiths (which he got attached to in May last year) this year, then gets into Covenant's sequel (Should Covenant be successful) while dropping out of one or both of the projects. Remember, he was also going to direct Blade Runner 2049 and The Color Of Lightening after Prometheus.
Also, the article in question mentions the sequel has already been written. So the big deciding factor is if Covenant is successful.
But I think it's safe to say we can expect Covenant 2 by May 2019/2020.