I'll be honest that I'm not a Clojure user, even with context I have no idea read that. Like every other language you have to be trained to read it correctly and is not intuitively obvious to everyone.
Parentheses take precedence.
The first symbol is the operator (add, subtract, sum, multiply, etc.).
Everything else is operated on from left to right.
(+ 2 5 7)
2+5+7
It's also not a Clojure thing. It applies to all Lisps or related like Scheme.
I have programmed python for about 20 years, and I still do stupid mistakes. I grooked all the r6rs scheme syntax in 20 minutes, except for (do ...) which somehow never sticks. It is also rarely used.
I have tried to evangelize scheme enough to know some people just don't like it, but for me it instantly clicked.
The problem I think is that not= returns true as long as any elements are non-equal, which means it is not analogous to +. I can't speak for clojure, but this is in line with all equality predicates in scheme, negated or not. A predicate that checked if any neighbours are not equal would in true scheme spirit probably be called not-any-neighbour-equal? :)