Either you were told incorrectly, or you misunderstood the statement.

On a OWE starting in BKK, you can take whatever flights you like within Europe, as long as there are only 4 of them (and no more than 2 between LHR and the faraway places, none of which are in your plans). Stopovers are not relevant, it's only segments that count in this case. (Note that you also buy 2 extra segments, which might or might not be a good deal given alternative means of getting where you want to go.)

It is possible that either you or your information source confused OWE tickets with OneWorld award tickets. For the latter, there is indeed a restriction that allows only 1 stopover in any city.