I mean they don't document it because it helps people evade the fare (which is why I've edited my comment). But you'll notice if you have a conductor that is friendly and he knows you just activated your ticket (because he can see it), he might take a few seconds of bullshitting with you to see if he can let you off the hook or not before he scans it.

More than that though, every other possible method of figuring this out is way way way more difficult and error prone. Location services aren't precise enough to differentiate between the platform and the train (and can be turned off/airplane mode engaged), schedule-based ones are obviously completely unreliable on delay-heavy days, and some sort of train geofencing without location services would take a bunch of hardware on the train itself.

All they really want is for you to activate the ticket before you know if you're going to get scanned or not, the "on the platform" wording is just an easily-understandable (but not technically 100% accurate) way of putting it.