I've said it before - sub numbers mean nothing next to engagement.
You can have 80 million subscribers and only engage 5% or have 100 subscribers and engage 40,000%.
A LOT of their engagement comes from non-subs who see their videos shared by those who actively engage in the channel.
If their average views are, say 4m per video and they lose 1m subs, that's effectively losing 25% of their reach wiht just those who engage. Now factor in those subs who share videos and non-subs engagement and you're losing a lot more engagement.
Sub numbers are so you can puff your chest. Nobody cares.
Performance and engagement is where you count your money - this is what "the man" looks at.
Remember, those who are unsubbing are considered those who engage because the engagement door swings both ways and we are seeing their active users fall by the wayside. I'm quite sure many of their subs aren't actively engaging.