Because I'm not a fool who buys into the corpo-marketing speak.
They very frequently say nothing about rumours or speculation - they just ignore it. That's the normal behaviour. Certainly this wasn't even on the same planet as an important news story that would move the stock price at the time, necessitating an immediate response, but they chose to speak publicly and make statements that it succeeded by their metrics.
I have no reason to doubt that, because the budget was quite modest, far smaller than most of Microsoft's games, it got great reviews, great community buzz, and it posted decent sales numbers on Steam for a game that launched day and date into a subscription service. As a point of reference, Forspoken was a much more expensive project which also had a day-and-date PC release on Steam, and they both launched around the same time, and Hi Fi Rush posted not just higher sales numbers but also outperformed it by revenue on the platform, no small feat when Forspoken had over 2x the RRP. People have also compared it earlier to Forza motorsports - another much more expensive game that Hi Fi Rush pulled more sales numbers than on the platform.
Why would Greenberg go out of his way to comment on a rumour just to tell a very direct lie? The most parsimonuious explanation is that Hi Fi Rush did pretty decently, met Xbox's commercial expectations, and exceeded their critical expectations.
We already had Schrier come out and clarify that one of the factors in shutting the team down was not that it was singularly unprofitable and unsuccessful, but simply that they were required to make harsh cuts and Tango Gameworks happened to be one of the farthest teams from releasing anything. Hi Fi Rush didn't do so overwhelmingly well that it saved the studio, but that doesn't mean it didn't do pretty well for what it was, a mid budget title.