Even the famously nimble WhatsApp which has fewer features and complexity needs this order of magnitude engineers so why would you expect Twitter to be leaner?
Whatsapp had 35 engineers for 450M users at the time of its acquisition by FB (2014). It had 50 engineers for 900M users (2015).
https://www.wired.com/2015/09/whatsapp-serves-900-million-us...
Just handling the ads, both backend and user interfaces for buyers, would quickly net you several more developers.
It's easy to have only 45 engineers when you constrain the scope of what they have to do very well, but the scope was much, much, much smaller than Twitter's.
In case you haven't noticed, twitter has many, many, many other features beyond direct messaging, each of which requires its own team to maintain, and many of which are far more complex than a messaging client.
I do not think Twitter had ever 8000 engineers - my understanding is that there were that many employees. It's easy to imagine a lot of non-tech roles, b2b sales, management, support etc.
I'm aware they scaled to 1B+ users with ~100 engineers but I'm failing to see why they'd be working on less complex problems.
Operating on almost all native platforms, at this scale, with those constraints, isn't something i find much easier than building a feed.
On Whatsapp the ordering guarantees are limited to within the thread and server side timestamps are good enough for the most part to enforce it. Whatsapp/messaging is a very horizontally scalable problem. With feed (assuming) you have a fully personal feed the storage alone is a pain to manage and optimize.
Now of course you can reimplement an ordering mechanism on top of an unordered one, with lost messages detection etc on the client side, and let the server be loose.
but that means reimplementing that logic on every client codebase.
We have evidence that 8,000 was far too many, because they're gone and Twitter is still Twitter.
We have zero evidence that 550 isn't still too many.
Mind you, I already had searches that triggered bugs in mobile app where first search would always fail, but things like significant drop in tweets I see in my feed from people I follow (and no, none of them were bots), random outages like Lists or the whole search feature just failing, pervasive feeling that even searches return maybe 1/10th of what I was getting before...
This is a tirefire.