I'd forgotten that trick - was something similar with Anglia, I used a few times in the evening from Liverpool Street on the Norwich service
It wasn't a trick - it was the standard approach to providing dining car services on BR from time immemorial, so Anglia inherited it form BR, and GWR still do it - allowing both classes to eat in the dining area subject to returning to their seat when the meal was finished - so if you were a standard or first class passenger you were meant to get up and go back to your respective seat in order to free the space for another diner.
Now I suspect that some staff allowed passengers to linger in their seats in order to save them the trouble of cooking and serving another meal to someone else....
and some passengers happily lingered in their seats as you got a first class set of the price of a meal.
I can recall an occasion where I had to 'suggest' to a steward that one of the people sitting in the dining car having clearly finished eating could be asked to 'move along' because I was hungry and wanted to order
Now many standard class passengers might not have been aware of this, but it was always stated very clearly in footnotes on timetables and the handy intercity timetable booklet about on trains catering and dining facilities (the Knife and Fork symbol)
and of course not many standard class passengers maybe wanted to 'fork out' on a dining car meal, presumably preferring the sandwich or burger from the buffet counter - despite in many cases the standard of the restaurant cars being better than many static restaurants I have paid handsomely to eat in, yet sometimes been unimpressed with the quality.
As I have argued on the forum before -
paying for a meal you actually want is a better system than giving away so called "free" stuff in 1st class - because with the former it has to be of sufficed quality for you to actually want to buy - with the latter the incentive is all aligned to the bean counter priority of reducing the cost as low was possible - hence where we are now on the MML with 'snack boxes' and sandwiches that are of such low quality that if they were on sale on the high street no one would buy them unless they were desperate (IMHO)...certainly not a product anyone who can pay for a 1st class ticket would be likely to buy through choice, surely.