I would say you are overstaffed if A) you can't afford them and / or B) you've grown so fast, you don't know how to make use of the staff you have. Perhaps there is a C) which is a variant of B) the business model you are running only requires so much staff and you hit a ceiling in growth.

I would not claim to have enough information about twitter being overstaffed? They might have a lot of technical debt, which can cause the need for more people to run it. But that is not "overstaffing".

The question is also how overstaffing is measured. If adding another person reduces the average productivity, you might define that as the cut-off due to the declining ROI for every other person added. But you probably also want some level of redundancy (aka reducing the bus factor) in order to account for the ongoing global health situation and general churn.

Of course repeated waves of mass layoffs, demanding a pledge of allegiance to the company and revocation of all employee privileges at the threat of being fired and publicly throwing your employees repeatedly under the bus are also ways to reduce churn, so the bar for "overstaffing" might have actually lowered.