Apparently this requires a hole to be punched in the sandbox to allow android apps to "impersonate" other apps (by way of signature spoofing).

The lineage folks didn't want to merge it in on grounds of security concerns.

(I'm not affiliated with the project, I just read the code reviews because I had the exact same question).

EDIT: for those interested: https://review.lineageos.org/#/c/64967/ and https://review.lineageos.org/#/c/65366/