Dictionaries seem to claim that while “brain” refers to the organ in the head, “brains” concern one’s reasoning capability, intelligence, intellect and so on. On the other hand, one of the most popular lines in action movies is “I’ll blow your brains out” and I seriously doubt whether this sentence could be turned into “I’ll blow your intellect out”. What do you say?
“brains” can refer to reasoning ability, but it can also be the plural of “brain” in the expected sense of two or more separate brains. Your example may be considered a set expression, but it also contains yet a third sense of “brains”, referring to the physical substance which brains are made of.
Sounds plausible… yeah, that’s probably the case.
It’s a bit like “his brains were splattered across the wall”.