I'm only just digging into the site, but some ONT pages (ex [1]) have information on how to set low-level parameters (MAC, various equipment IDs, etc). Probably won't get you free HBO, more likely to get your ONT banned at your ISP, but maybe you'll get free internet before that.
This isn't about getting free internet, no competent ISP will let the link come up without a serial number registered with the port. This is about bypassing the awful gateway hardware many fiber ISPs mandate.
There are also folks that want to overwrite the MAC, serial, etc to clone their ISPs ONT - allowing them to use a different GPON/XGSPON ONT/SFP(+) module [0].
[1] https://hack-gpon.org/ont-nokia-g-010g-t/#gponomci-settings