The build server is not purely free from proprietary blobs
According to https://f-droid.org/docs/Installing_the_Server_and_Repo_Tools
In order to build Android apps with the fdroidserver toolchain, Java, the Android SDK, and some other essential tools must be installed. Only parts of the Android SDK are available in Debian, so the Android SDK must be installed manually, as well as the packages that it requires (the Android SDK tools include some 32-bit binaries, so even 64-bit systems need these i386 library packages). The F-Droid tools use the Android SDK to build and inspect apps, so you must have the Android SDK installed and setup before using fdroidserver.
According to https://developer.android.com/studio/terms.html
3.4 You may not use the SDK for any purpose not expressly permitted by the License Agreement. Except to the extent required by applicable third party licenses, you may not copy (except for backup purposes), modify, adapt, redistribute, decompile, reverse engineer, disassemble, or create derivative works of the SDK or any part of the SDK.
Therefore, the build server is not purely open source. A purely open source Android SDK should be rebuilt from source.
Well but this is less than an issue, because building the SDK costs a lot of CPU cycles and bandwidth.
Thank you very much.