Can't link to the posts directly to demonstrate since the spoiler overlay is only shown in the feed. It's assumed if you click into the post you want to see the spoiler. I suppose I could change that, but the behavior aligns with how the NSFW tag works.
To give an example of how it works in the feed, here's a search link that should work (it's what I used to make the post screenshot): https://tesseract.dubvee.org/search?type=All&q=posts%3A%5Bspoiler%5D
Im guessing like abcd [spoiler] efg would trigger it too?
Correct. While that wouldn't turn into a flair (only [flair text] at the beginning or end of the title will be used as flairs), it will trigger the spoiler overlay if [spoiler] appears anywhere in the title. I did this consciously to account for inconsistencies in some of the spoiler usage I've seen. It could easily be changed to only trigger on actual flairs (at the beginning or end of the title).