It makes perfect sense because when Jack was part time CEO of Twitter he was known to spend less than a full work day at the company per week. Productivity tends to trickle down, it's contagious.
Much like a super high performer on a team tends to lift 1x engineers on the team to 1.5x - 2x engineers, I've observed.
Parag Agrawal as a CTO shipped essentially zero features. In fact, Twitter got noticeably worse over the years. Periscope and Vine were genuine innovations, both cast to the side.
A lot of Twitters headcount was specifically devoted to serving certain regions. For example, Twitter had integration with multiple different game services and specific regional functionality. They had a large API that they had to maintain, update and respond to dev requests.
A lot of what happens isn't visible.
The fact that Twitter hasn't gone completely offline in a fireball as predicted over and over by these same people itself proves they were redundant.