You're right to use "indices".
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.
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.
Huh, I do use indices for 2+ index. I guess I’ve been wrong. I’m surprised nobody has ever said anything in my nearly 50 years.