So, according to this it's just LineageOS with a preconfigured system-wide Adblocker. Points for being honest about it.
These Adbloclers exist in F-droid, easily installable.
Shipping devices with those preinstalled & preconfigured is worth some money (remember, the vast majority of people have never installed an operating system), but it all seems a little steep.
OTOH I had a look and the markup for the Fairphone is less than 100€ (439€ directly vs. 519€ from iodé) - maybe that's the intended audience.