Sebastian Seung stated plainly in his most recent book that he fully expects to die. “I feel quite confident that you, dear reader, will die, and so will I.” This seems implicitly extremely skeptical of current cryonics techniques, to say the least.
Irreversibility is not a timeless concept; it depends on currently available technology. What is irreversible today might become reversible in the future. For most of human history, a person was dead when respiration and heartbeat stopped. But now such changes are sometimes reversible. It is now possible to restore breathing, restart the heartbeat, or even transplant a healthy heart to replace a defective one.
Semantics are tricky because “death” is poorly defined and people use it in different ways. See the post and comments here: http://www.geripal.org/2012/05/mostly-dead-vs-completely-dead.html.
As Seung notes in his book: