This is the 2189th most frequent Chinese word.
早就
"早就" means "long ago" or "already for a long time."
早就 is used here to indicate the speaker knew the news well before the present moment.
我早就知道这个消息了。
I already knew this news a long time ago.
In this sentence, 早就 emphasizes that the completion of work happened much earlier than now.
他早就完成了工作。
He finished the work a long time ago.
The usage of 早就 in this case implies that the action of departing was expected to occur earlier.