why do you think the previous management team couldn't pull an Elon and fire 80% of the engineering staff themselves? why they needed an external leadership to take over and do it?

Part that i can't wrap my head around was at least in case of twitter, it was a hostile take over. In case of Vimeo, it didn't look hostile at all.

I wonder if existing management had a lot of social ties to the existing workforce so that they couldn't easily get away doing that themselves without a big hit to their reputation.

Letting someone else do the dirty work allows them to disassociate themselves from the (predictable) outcome and frame it as just business.