1) very likely the OP's experience was indeed caused by a bus driver who dropped the passengers at a wrong (Schengen arrivals) entrance to the terminal
2) as somebody upthread remarked: how often do passports get checked by the French when taking a (car) ferry from the UK to France these days? In my experience: never
3) the Dutch immigration authorities still use common sense (mostly). A friend of mine, Indonesian passport, who had spent his holidays in the UK (UK visitors visa) and missed his connecting flight back to Asia at AMS (and had to wait 24 hours) was given a short-term visa on the spot by AMS immigration
4) in the old days (before Schengen) my US friend who visited AMS regularly always told me that his passport had *never* even been opened at AMS - just waving the cover did the trick.
5) if you know something about risk management and limited resources it makes sense to be a bit lenient with checking passengers off a flight from Southampton!