Reddit's own guidance on the topic says the presence of profanity could cause a post or a sub to be NSFW.
A moderator toggling their sub NSFW due to the presence of profanity would be fully in line with their own guidance, and I'm pretty sure r/shittylifeprotips had a little profanity. (Hint: it's in the name.)
Reddit is just desperately trying to interpret their rules in whatever way they desire because they're panicking at losing revenue.
But it comes in bigger cans, so you can spray longer. A reasonable tradeoff under some conditions.