Lemmy doesn't do spoilers the same way that Reddit does. Sync appears to still be using the reddit style of spoilers, i.e. >!spoiler!<, however on the web-ui those are visible to users, and the spoilers that web users use in the editor which look like this:

description
spoiler goes here

Aren't hidden in Sync. Consider this a PSA for users of Sync that may want to avoid spoilers or avoid spoiling others. Personally, I'd like to see >!spoiler!< supported more widely on Lemmy, but in the meantime this should probably be addressed in Sync.