It's basically a desktop as a service, bound to lightweight, portable device that can be replaced as needed without the fuss of reinstalling, restoring stuff from backups etc. It's different from ordinary desktop where emphasis is strictly on on-premise HW and installations with cloud services supporting some workloads. If I interpret it correctly, Solu is first and foremost a cloud service with on-premise/portable HW supporting the use of those services.

However, the question is, will it be enough to have only a portable computer and if not, is it even possible to pack enough performance in the form of smart-phone to use it (without making it too bulky, ugly and restricted because of battery life). So far nobody has really been able to do it successfully.

True, however, doing it with dedicated HW platform instead of trying to sell it as a service on top of existing desktop environments would be very, very difficult. Current way is more effective way to offer real alternative instead of just trying to augment something already existing in restrictions set by those platforms (not to mention compatibility-issues when trying to support multitude of different setups).