> Because Java starts treating everything as a String once it has found a string in System.out statement

Uh, no, I suspect this is an example of "magical thinking" by the author.

Far more logical theory: It's working left-to-right, doing int-to-int addition until it hits the first string, which is "=", at which point it is locked into doing string-to-string concatenation for the remainder.

String concatenation is one of the few places Java has overloaded operators, and this quirk (and reader confusion!) is an fair example of why Java does NOT allow users to overload operators for their own classes.