There are two big pieces of evidence that this is a hardware announcement. First, Google is a heavily compartmentalized company, and the person promoting this event on Twitter is none other than Risk Osterloh, Google's senior vice president of Hardware. Osterloh is behind the division that brought us the Pixel phone, Google Home, and every other Google hardware product. His involvement is a solid sign that, yes, new hardware is coming.
The second big piece of evidence is that Google is also hosting a teaser page on store.google.com. The Google Store. The Google Hardware Store. Google could host a teaser page at any of its thousands of sub-domains, but for some mysteriousreason, it picked the Google Store. The "Google Store" doesn't sell software—that would be "Google Play." The Google Store only sells hardware—things like smartphones and tablets and laptops and Google Home speakers. Soon, apparently, it will sell video game hardware.
According to a report from The Information (subscription required), Google's gaming hardware was reportedly codenamed "Yeti" and has been in the works at Google for over three years now. As pointed out by Tyler Choiniere on Twitter, On the Google Store's teaser page, the background video is titled "yeti-teaser" in the source code.
You can read the full thing on Ars Technica.
Do note (and Ars itself notes this), a new gaming console can still just be a "streaming box" (a hardware receiver for Google's streaming service).
That said, one way or the other, it seems like Google is gearing up to announce a brand new console in some form or the other next week.