It's because we wanted our team to be aware that we'd have this intention so we don't miss out! If someone leaves PostHog voluntarily to start a new company (i) that's sad for us but is a pretty cool reason to leave and (ii) if we were willing to employ them, we'd be willing to invest in them more.
There are some pieces of missing context I guess. Primarily that we're small still (30 people) so we tend to hire a lot of ex founder / generalist type people (and most are technical) who'd be quite well suited as founders.
One thing I will do is add a more detailed handbook entry to create more clarity around this benefit so we don't mislead/confuse. Issue made (https://github.com/PostHog/posthog.com/issues/4013)!