It gets a bit complicated when you have many authors all contributing to a github repo. Some projects require the authors to sign off so that the copyright is assigned to a single entity, that makes relicensing easy. With an open source project like Zig it might be complicated to relicense since you have many contributors.
It seems that Zig should maintain a contributors file since they have chosen the joint copyright model with the MIT license. They could then refer to that in a copyright notice, which could be a single line per file.
On the Zen side however there seems to be a gross misunderstanding of copyright. Just because you make some changes to a source file (or no changes at all) does not allow you to completely reasign the copyright to yourself.