So you were going from Brussels to London - in Brussels you would have been stamped out of Schengen by a Belgian officer and then stamped into the UK by a British officer based in Brussels. This is because the UK operates a system similar to US preclearance for the cross-channel trains.
So even without a UK stamp, the French officer should have had two other Schengen stamps to see after your Helsinki entry in June - when you departed Brussels in June/July(?) and and when you re-entered France(?) in August(?). I know officers from some Schengen countries didn't bother to stamp US passports in the past, but a stay from June to August is perfectly fine anyway (at least, as long as you had not been in Schengen since ~April).
That's correct. It was easiest to tell the French officer "no, I left the EU X number of days later and just reentered on ___" because that was a direct and simple answer to his question.He looked down and saw the British Eurostar indicator and that was that. (Also, probably since I spend so much time in the UK and never had a passport stamp, that train stamp was most etched in my mind so many weeks later, so it was probably the first thing I thought to say, as well, now that I think of it).