There are lots of reasons for him to lie. Xbox has been having a really bad round of PR for the past few weeks; reporting of HiFi failing to meet expectations would just compound on the slew of other bad news that has come about. And would be yet ANOTHER Zenimax game not "living up to expectations" (others being RedFall reception, and Ghostwire performance on Series X vs PS5 despite an extra year of development).
There's also the fact that HiFi comes from Tango, MS's sole internal Japanese studio, and there's the ever-running narrative of Microsoft just being terribad at courting Japanese developer support. If they want to retain at least respectable optics in that regard, they don't need reports of an otherwise refreshing nugget of a game from their internal Japanese team, failing to meet internal sales/revenue expectations, even if there's a chance said report(s) could be true.
Also not to mention, it's news that could reflect poorly on Microsoft in terms of management and/or internal expectations of acquired publishers...when they're currently trying to acquire yet another massive publisher in ABK. Would not really reflect well there.
I'm not saying any of this to state definitively that Aaron Greenberg is lying, BUT it doesn't dismiss the possibility he is. Not to mention, this guy has been very disingenuous and misleading in multiple instances in the past on Twitter, so I wouldn't say he's the most blindly trustworthy mouthpiece to listen to.