Sure, but custom integrations seem unlikely to explain the majority of Uber's technical headcount. Let's say they average a dedicated engineer for each of their 1000 largest markets/locations. Let's assume another 200 across the countless smaller markets. Let's assume 50% overhead atop this for things like infra, tools, and management. These all seem like exceedingly generous estimates to me.

They actually had 5,000 engineers in the tokenmaxxing blog post. That's a lot of engineers for the rest of Uber's business activities.