ES support confirmed that they are effectively the same; I think I mis-parsed the quote from the blog post above. However, it is possible that a shard could be in-sync but not active e.g. when a green index has been closed.
A shard copy can also be in-sync but not active if it's unassigned (e.g. the node holding it has shut down) but nothing has been written to that shard since the last time that copy was active. The converse is not possible, however: all active shards are necessarily in-sync.
a single in-sync copy is sufficient to serve read requests
I think that's wrong, the copy has to be active to serve read requests. If it's closed or unassigned then it obviously can't serve searches, even if it's still in-sync.
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