Penalty fares can only be issued under certain circumstances.
One of the preconditions (among others) is that the station must be within a Penalty Fare scheme. A station that is within such a scheme will be a Penalty Fare station for that purpose. That does not necessarily mean a passenger starting their journey there can be charged a Penalty Fare.
For example a Penalty Fare can't be charged when travelling with a company whose trains are outwith the Penalty Fare scheme, nor if the next station is outwith the scheme (e.g. London St Pancras to Leicester), nor can anyone other than an authorised Penalty Fare collector make such a charge.
Such as King's Cross?
Problem is it's not that simple... as that would only apply if travelling with GTR, of course!
And, while VTEC may require you to buy a ticket before boarding their trains, that doesn't mean they can issue a Penalty Fare.
This applies whether or not the station is a Penalty Fare station though