But it's not equivalent. At St Pancras, Dover, (and I assume Folkestone although I've never travelled on LeShuttle), you go through both borders before you even get on the train - leaving you free to just casually walk away at the other end without going through any kind of security or passport control. The same is true in the other direction.
So this morning we have lots of people who have already "arrived" in Schengen and "left" the UK despite them sitting in the St Pancras lounge, or on a LeShuttle train which has not left the platform at Folkestone, or on one of those trains which departed but never made it into the tunnel before turning back.
Meanwhile there are people in Paris, Brussels, Amsterdam, etc who have "entered" the UK and "left" Schengen despite still being in those cities.
In airports you go through border control to "leave" the country, but don't "arrive" at your destination until you have passed through another border control point in that airport. Between the two border points you are essentially in no-man's land. If your flight is cancelled after passing through, you can simply enter again because you haven't "arrived" anywhere else, using the airport's already setup inbound passport control.
St Pancras and Dover (etc) are not setup in this way - there is a clear exit with no obstacles, so you will not pass through any kind of border again (leaving you still "in" Schengen) unless there are physical alterations made to the routes through the building to essentially reverse the flow.