It appears the company is pretty serious about trying to create an open market for the expansion cards. They have CC-BY licensed reference designs of the expansions cards (https://github.com/FrameworkComputer/ExpansionCards), and employees are in the forums answering questions and giving pretty forthright design advice (eg, https://community.frame.work/t/dual-usb-c-expansion-card-moc...). It also appears that they're giving funding to people developing them (eg, https://community.frame.work/t/rp2040-microcontroller-expans...).

I was enormously skeptical when I first heard about the company, as the history of computing is littered with the devices and prototypes of startups that suggested they were going to make something different and open and extensible, only to have those promises be completely overblown, and usually have communication become shady and vague. But while I'm not quite confident enough to buy one yet, and I'll be very interested to see how their ecosystem evolves over the next few months, it actually looks like they might be succeeding here.