I can't help but notice that there's no license file in the repo, nor is there any embedded notice in source code (except the MIT licensed .golangci.yml). The only mention of the license of the project is the License badge in the README.

I'm not a lawyer, but according to the Apache 2.0 license:

[...]
c. You must retain, in the Source form of any Derivative Works that You distribute, all copyright, patent, trademark, and attribution notices from the Source form of the Work, excluding those notices that do not pertain to any part of the Derivative Works; and
d. If the Work includes a "NOTICE" text file as part of its distribution, then any Derivative Works that You distribute must include a readable copy of the attribution notices contained within such NOTICE file [...]

Again, not a lawyer, but doesn't the LICENSE file and any other copyright notices need to be retained in a derivative work?