If you're telling people the site, thus in its entirety since there is no specification otherwise, is 18+, it should be expected to be 18+ and that is what will be seen.The difference is that this is is not a porn site. This is a writing site. People also go on things like Reddit, Facebook, etc. that can have NSFW text / images but that also take precautions to make sure these are appropriately hidden behind warnings.
The disclaimer at the bottom of the site is to show that the site is for 18+ due to housing adult content, not to state that porn will be unexpectedly posted or sent.
It's not difficult to understand that or at least change the wording on the landing page to be less contradictory. This also fully applies to all other places, whether Reddit or elsewhere, which have an 18+ warning but then self-contained sections for exactly that; it is not exclusive to BMR, and I'd call those other places out, too. It is the definition of contradictory.
Anyone who sees that footer notice should not be entering the site if they care about seeing such content, when the warning is clearly in front of their face. If you're going to make rules like this, at least don't half-measure it, and do it properly in a way that's not contradictory or hypocritical.