You are quite right.
As has been pointed out here recently, the system as a whole may achieve 100% availability by
using pervasive redundancy but the individual machines in the system won't. The associated PDF
suggests that there may be long lived processes that would be intolerant to any outages due to
their inability to save state properly or would exhibit a certain unhappiness with network
connections dying. One would suggest that a system administrator designing and implementing a
HA environment would make sure that each and every piece of software on each machine could
handle a reboot if necessary.
While I love the idea of hot updating and while I think this implementation is a fine fine
hack worthy of much admiration I would welcome a concrete use case other than, "look I have 5
years, 2 months, 4 hours and 16 minutes uptime".