It's not a native English plural, it's Latin straight from the nominative plural <indicēs>. You will find that <indexes> is far more common in English. The supposedly "correct" plural, <indices>, is something that has to be learned through education, typically.
There are a lot of Latin/Greek terms (typically scientific) whose plurals must be similarly learned because otherwise native English speakers will use the simple plural -s suffix with no concern whatsoever.
The example is just provided to show how inconsistent plurals are in English. 90% of the time you just slap an "s" on the end of a word, but there's a crazy amount of exceptions to that rule.