I believe they are effectively going all-out, with both the already tested streaming service and a local-processing console. The Sega partnership could be huge, and Google has the resources to poach several heavyweights.
IIRC, there hasn´t been a comparable scenario: four companies haven´t been simultaneously successful in the console space. The current fragmentation of the PC gaming stores adds further complexity to the near future of the gaming industry.
A newcomer at this point of history is indeed surprising, but I think they have some vital advantages when compared to the current platform holders: they own YouTube, Chrome and Android, which are almost worryingly dominating and will be presumably integral to their strategy. Their ecosystem is vast and relatively coherent. They (sometimes undeservedly) have a mostly positive image, mainly among the non-tech savvy.
They somewhat stole Microsoft´s thunder about streaming. Next Tuesday both companies will be giving more details about their respective services, but the spotlight is undoubtely on Google´s side. If Google´s project takes off, I think this could lead to a paradigm-shifting battle, with much more emphasis on the cloud and monthly subscriptions.
They have much more resources than Sony and Nintendo, both of which have strong and vocal userbases but (as far as we know) will probably try to remain tied to more traditional, hardware-based, closed ecosystem offerings for the foreseeable future.