I think Lichess is per design not a chess database. It is a playing app, a good one.
It collects and stores only the games that have been played via Lichess. So I guess even with that search function implemented you won´t find that particular game. But I might be wrong.
It is true that Lichess isn’t designed as database, but it collects and stores NOT only the games that have been played via Lichess, but it has its Masters database, and also stores those games which you enter through Analysis board, and those which Lichess broadcasts, like the mentioned Oslo Esports Cup (Meltwater Champions Chess Tour)
Therefore it does function as a database, and needs this search field. I can't imagine why it would be harder to implement ECO search field than, for example, completely unnecessary fields about the duration of the game.
Oh, I was not aware. I agree - who needs to filter for the game duration? Strange.