That’s two words. Which, given the source, isn’t surprising.
Hmm...it’s two words used as one. There are plenty of other examples.
Two words used as one does not make for a valid Scrabble word.
“folic acid” is in the top 4 best-selling dictionaries in the USA but “folic” by itself is not. Thus, folic is not a valid Scrabble word.
Now you do have a point. If “brain rot” were not in the dictionary, most would be able to figure it out.