Prior to June 2023, Lemmy was holding steady around 1,000 monthly active accounts. So it's still up more than 30x from that baseline.
Since this graph shows active accounts and not active users, I'll bet a significant contributor to the drop is people not using all the alt accounts they initially created.
Why did it had to be the peak, though? We are talking about a time where "major" protests were being taken against a platform with more than 400 million monthly active users. Taking only 100k as the "peak" is already pathetic, now you are telling me that we should be happy to keep a third of that?
That's completely normal for literally any new platform. You saw the same thing with Threads, down to the hordes of people proclaiming it dead after the initial wave of interest calmed down.