Given how under developed they are, they do seem like a last minute addition or change.
Given a number of factors, including how the other three Allied races tie into Legion's story, it seems like Krokul were likely the original Allied Race in place of Void Elves. Think about it, Blizzard made new models and deliberately added them into Argus. If there weren't any there, no one would have cried foul, because up until 7.3 we didn't know there were OG Eredar still living there. They're also specifically Broken, which has been a reasonably popular sub race choice back when people were talking about sub races.
Now getting new Broken models would make sense if they had a heavy story influence. But after they're set up, they don't really contribute much. They're just sort of there while the armies of Azeroth and the Army of the Light lay siege. So we get brand new Broken models, a new faction of them is set up and don't really add much to the overall story, and they were added as a new reputation. Given how horribly under developed the Void Elves are, and given Blizzard gave time to set up another race that has yet to go anywhere, it seems like these Broken were likely one of the original Allied Races Blizzard was planning for pre-release. And were put on Argus so they have more story significance than the ones residing on Outland.
Now that's not to say we won't get Krokul, there's a good chance we'll still get them or another form of Broken. But it does seem that Krokul are being set up that way, and if they were in place of Void Elves they would tie in a lot stronger both narrative wise and gameplay systems wise. We get Void Elves from Argussian Reach rep - which is weird to say the least - it would have made more sense if it were Krokul instead. They have new models, an area that can be phased for a starting zone and the perfect timing story wise.
The reason they likely changed their minds later on was they would have been doubling up on Draenei sub races during the Allied Races' first implementation.