The Outer Worlds exclusivity came as a shocker to another community of fans who were disgruntled after it moved to Epic following a period of marketing on Steam. It turns out that maybe not even Obsidian Entertainment was aware of the move.
The Outer Worlds turned out to be one in a bunch of new titles exclusive to Epic Games Store, which subsequently prompted frenzy among the fans. Most are content with blaming Epic Games for snatching these games while others are including Obsidian Entertainment and Private Division in the whole affair regarding the misleading of fans.
Now it appears that maybe Obsidian could be excused from the exclusivity kerfuffle as at least a part of the company was not aware of the timed exclusivity with Epic. The developers were for The Outer Worlds just four hours before the exclusivity announcement became official. It doesn't really make much sense to add Steam achievements for a game that is one year away from release on the store.
This hints to two possible outcomes - either Obsidian Entertainment's staff was not aware of the exclusivity and proceeded to prepare for a Steam release as they normally would or they were aware of the switch and intentionally misled the fans.
The latter would not make much sense as some potential customers were inevitably going to be furious by the announcement, so aggravating them further with intentional misdirection would only add fuel to the fire and would serve no benefit at all.
Obsidian staff being in the dark is the more plausible scenario here as the same story happened with Metro Exodus - Deep Silver called the shots and 4A Games of the decision making, at least not entirely. The publisher made the choice to jump ship.
On the other hand, it is still quite possible that key people in Obsidian knew about the switch but chose not to make the information public. Keep in mind that the game will be available on Microsoft Store too, so the company might be attempting to pull a manoeuvre similar to that of Ubisoft, which resulted in a in sales directly from their store.
Private Division, Microsoft and Obsidian Entertainment didn't release any public statements about the switch so far so take the rumours with a grain of salt for the moment.