I don’t know where you read that but it is wrong, the only thing you will be sold at a station is a full fare ticket if there are any seats left.
If you are going to buy a full fare ticket it is best to buy that online from eurostar.com.
There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.
The best place to see the availability of passholder seats on the Eurostar is
https://www.b-europe.com/EN/Booking/Pass#TravelWish
Maybe Al knows current situation-or find the /en/ forum railforums.co.uk -INternational-had a very recent overveiw of nearly same point: as I remember the counter of it at St Pancras is CLOSEd for now.
Another thing is that these trains are suddenly so much in demand and several were cancelled, that any hope on spare seat is like hoping to win the 10 million i the lottery.
Flix or fly is the best other option.