While I agree it's silly to think musk is going to fire everyone, Whatsapp hit 1 billion messages a day with only 11 engineers. I certainly think twitter could function at a base feature set with far fewer than 2k.
I get that most view it as a proxy for business growth, but I fear there's a growing group of non-technical investors/leaders/founders who see FAANGs with 4000 engineers, so you're only successful if you have that many too.