At more than 8 times the published maximum fine: I'd say this smacks of being a scam. See if you get any more threatening reminders, if you do: take it to a lawyer and they'll do the checking for you.
If you are at a given altitude and can see the other aircraft below you, then yes, they are that low. However, as a drone pilot too, it is not that easy to see an airplane that is below you in the view finder of your phone or Smart Controller...