Seasonality doesn't mean all years are the same. The mean (the average) doesn't mean there are no differences and huge fluctuations.
If the same pattern were repeating itself, all would buy and sell at the same time, but wait... somebody has to be on the other side, too. There can't be a clear and strong seasonality because in this case all wanted to take advantage of it. And the anomaly disappears or turns negative with the transactions of the crowd.

There are seasonality in stock markets, but not so big to offer an arbitrage possibility. I suppose the same will happen in crypto markets on long term.