Why is an if/else if/else for a simple boolean not giving an “unreachable code” error

From JLS 14.21. Unreachable Statements

It is a compile-time error if a statement cannot be executed because it is unreachable.

and

The else-statement is reachable iff the if-then-else statement is reachable.

Your if-then-else statement is reachable. So, by the definition the compiler thinks that the else-statement is reachable.

Note: Interestingly the following code also compiles

// This is ok
if (false) { /* do something */ }

This is not true for while

// This will not compile
while (false) { /* do something */ }

because the reachability definition for while is different (emphasis mine):

The contained statement is reachable iff the while statement is reachable and the condition expression is not a constant expression whose value is false.