Basically a NUC with no case. I guess 3D printing your own case is the "hackable" part.
Presumably also the Arduino-style GPIO, pin-out docs [1], and just overall detailed docs like the power supplies page [2]. Good docs and schematics were not unusual forty+ years ago but are probably fair to describe as a “hackable” feature today.
[1] https://docs.lattepanda.com/content/sigma_edition/IO_Playabi...
[2] https://docs.lattepanda.com/content/sigma_edition/Powering_O...
Home computers of 40+ years ago had complexity comparable to today's Arduino boards, and often had GPIO-like extension ports.
In the first place, it was possible to document them succinctly enough.
In theory, I would love to pay for a device which is well-documented and provides a repair and modification friendly interface at all levels of hardware and software abstraction but in practice I’ll probably buy a macbook air because they mostly just work and the air is cost competitive vs the “open” niche.
And I don’t know about System76 in particular but my overall impression of binary blob driver acceptance is that the open folks lost that battle which narrows the gap with Apple tech.