Hi J! You just don’t want to directly eat the seeds; they are strained from the vinegar (and also in the candy recipe, are heated/cooked).
It’s surprisingly difficult to extract cyanide from seeds; here’s a conversation about that I found interesting:
https://chemistry.stackexchange.com/questions/24528/how-do-i-extract-cyanide-from-apple-seeds
What you could do though is simmer the berries in the vinegar for 5 to 10 minutes on lowish heat, or can the finished vinegar (water bath for 10 minutes). Then the berries/berry vinegar is heated/cooked & you don’t have to worry about it either way. :)