Translation:They had not met each other that evening.
And why is "They had not known each other that evening" incorrect if "know each other" is even shown as a possible translation in pop-up tips here?
On that site I could only find: " 'conoscersi' found in these entries Inglese: get to know" Could you please let me know where you found the "met" reference? Thanks.
There are two tabs, one for Word Red and one for Collins. Click on Collins.:l'ha conosciuto all'universita'
It makes sense to say that had not met each other that evening because you could be talking about them meeting a different meeting or that they did not do something together that evening because they hadn't met, so then you would clarify. There is nothing wrong with this sentence.