Correct. To get around that limitation, I have seen users on other communities mark a post as NSFW and then include in the title something along the lines of "(NSFW for spoilers only)". Obviously this is far from ideal, as some users/instances block all NSFW posts, and some others set NSFW posts to not be blurred at all.
Yeah, my home Instance doesn't allow NSFW material so I could neither create such a post myself or even view it without logging into another account (not ideal for a moderator).
Best bet is just putting [SPOILER] in the headline (without any spoilers images) and/or use spoiler tags appropriately in the text depending on what the OP wants to post about.