In contrast to companies like Google, Apple, Microsoft, Amazon etc. that have mountains of their own money to burn (rather than investors') on research and OSS side-projects, it always seemed to me that Uber was trying to play the same game, but far too early. Paying lavish SF engineer salaries to generate cool, but not revenue generating, software is probably excellent for morale, culture and recruiting, but a dubious use of resources when you are losing money seemingly faster than it would be logistically possible to literally burn it.
Saying they're ~ "culling the low performers" can be entirely true, but it is also a Silicon Valley, meritocracy-culture-friendly way of saying "we're losing far too much money to pay bloated growth-stage poaching-game salaries to engineers, so if you're not working on something that generates revenue, glhf"