Inability to use WebSockets/long-polling is the primary reason I haven't been able to use FaaS as my API layer, so this is great news. I would say the primary barrier I see remaining is db connections for RDBMS. Where a normal persistent environment might just use connection pooling, and indefinite amount of lambda instances connecting to a DB requires a separate instance running something akin to PGBouncer. It's possible that this is simply the long term solution, but prior to this announcement I would have said that bi-direcdtional communication with a client on FaaS architecture was not going to be possible, so I'd be interested to see if an alternative service or bit of functionality crops up.
This seems to be very similar to the Pusher Model of a WebSocket Proxy. It fronts the WebSocket connections for you and forwards all the messages to your HTTP endpoints. It will be interesting to see if AWS implements things like presence channels, etc.
You could implement presence on this, as it will send events on connect and disconnect.