This is true, but there's not much research on how quickly methemoglobinemia affects the brain. The studies I reviewed don't mention brain damage, except to say that timely treatment with methylene blue is imperative in preventing brain damage. In the available studies of people who have recovered from acute methemoglobinemia, there are no reports of brain damage, likely because brain death would occur about 10 minutes after the brain being derived of oxygen.
All this to say that while SN can potentially cause brain damage, it is far more likely to kill you than leave you with any permanent damage. Methylene blue does an exceptional job of restoring oxygen levels to normal. I would never say there's not a chance of something happening, but significant permanent damage is extremely unlikely, as has been documented in the research.
As for long-term effects, of course we don't have any longitudinal studies on SN to date. We'll need to wait to learn more about those.