Optimistic updates are great for inconsequential stuff like an HN upvote. The user likely doesn't even want to see an error message if their upvote timed out.
But it definitely is "lying to the user", or better phrased "breaking user expectations". It's just that the user isn't likely to care except for important actions.
For example, imagine applying optimistic update to sending an email. The user would expect to be able to close their laptop after sending an email and seeing the UI transition. It would be catastrophic if the email was never actually sent.
Optimistic updates are best relegated to micro-polish imo. Frankly, I think it's overused especially in combination with autosave UI.
Gmail does this all the time, and it makes me mad. I just sort out some mails to have my empty inbox, and then close the browser tab. And it says that the changes haven't been saved, so I need to deal with the popup, and then close the browser tab again...
