LineageOS isn't unsigned, it just happens to be signed by keys that are not "trusted" (i.e., allowed - thanks for the correction!) by the phone's bootloaders.
thats effectively the same thing.
The whole point of the majority of PKI (including secureboot) is that some third party agrees that the signature is valid; without that even though its “technically signed” it may as well not be.
not allowed is a clearer language here.